refactor(env)!: enhanced environment variable substitution (#2490)

# What does this PR do?

This commit significantly improves the environment variable substitution
functionality in Llama Stack configuration files:
* The version field in configuration files has been changed from string
to integer type for better type consistency across build and run
configurations.

* The environment variable substitution system for ${env.FOO:} was fixed
and properly returns an error

* The environment variable substitution system for ${env.FOO+} returns
None instead of an empty strings, it better matches type annotations in
config fields

* The system includes automatic type conversion for boolean, integer,
and float values.

* The error messages have been enhanced to provide clearer guidance when
environment variables are missing, including suggestions for using
default values or conditional syntax.

* Comprehensive documentation has been added to the configuration guide
explaining all supported syntax patterns, best practices, and runtime
override capabilities.

* Multiple provider configurations have been updated to use the new
conditional syntax for optional API keys, making the system more
flexible for different deployment scenarios. The telemetry configuration
has been improved to properly handle optional endpoints with appropriate
validation, ensuring that required endpoints are specified when their
corresponding sinks are enabled.

* There were many instances of ${env.NVIDIA_API_KEY:} that should have
caused the code to fail. However, due to a bug, the distro server was
still being started, and early validation wasn’t triggered. As a result,
failures were likely being handled downstream by the providers. I’ve
maintained similar behavior by using ${env.NVIDIA_API_KEY:+}, though I
believe this is incorrect for many configurations. I’ll leave it to each
provider to correct it as needed.

* Environment variable substitution now uses the same syntax as Bash
parameter expansion.

Signed-off-by: Sébastien Han <seb@redhat.com>

tests/external-provider/llama-stack-provider-ollama/run.yaml

@@ -1,71 +1,101 @@
-version: '2'
+version: 2
 image_name: ollama
 apis:
+- agents
+- datasetio
+- eval
 - inference
+- safety
+- scoring
 - telemetry
 - tool_runtime
-- datasetio
 - vector_io
+
 providers:
   inference:
-  - provider_id: custom_ollama
-    provider_type: remote::custom_ollama
+  - provider_id: ollama
+    provider_type: remote::ollama
     config:
-      url: ${env.OLLAMA_URL:http://localhost:11434}
+      url: ${env.OLLAMA_URL:=http://localhost:11434}
   vector_io:
   - provider_id: faiss
     provider_type: inline::faiss
     config:
-      kvstore:
+      metadata_store:
         type: sqlite
         namespace: null
-        db_path: ${env.SQLITE_STORE_DIR:~/.llama/distributions/ollama}/faiss_store.db
+        db_path: ${env.SQLITE_STORE_DIR:=~/.llama/distributions/ollama}/faiss_store.db
+  safety:
+  - provider_id: llama-guard
+    provider_type: inline::llama-guard
+    config: {}
+  agents:
+  - provider_id: meta-reference
+    provider_type: inline::meta-reference
+    config:
+      agents_store:
+        type: sqlite
+        namespace: null
+        db_path: ${env.SQLITE_STORE_DIR:=~/.llama/distributions/ollama}/agents_store.db
+      responses_store:
+        type: sqlite
+        db_path: ${env.SQLITE_STORE_DIR:=~/.llama/distributions/ollama}/responses_store.db
   telemetry:
   - provider_id: meta-reference
     provider_type: inline::meta-reference
     config:
-      service_name: "${env.OTEL_SERVICE_NAME:\u200B}"
-      sinks: ${env.TELEMETRY_SINKS:console,sqlite}
-      sqlite_db_path: ${env.SQLITE_STORE_DIR:~/.llama/distributions/ollama}/trace_store.db
+      service_name: "${env.OTEL_SERVICE_NAME:=\u200b}"
+      sinks: ${env.TELEMETRY_SINKS:=console,sqlite}
+      sqlite_db_path: ${env.SQLITE_STORE_DIR:=~/.llama/distributions/ollama}/trace_store.db
+  eval:
+  - provider_id: meta-reference
+    provider_type: inline::meta-reference
+    config:
+      metadata_store:
+        type: sqlite
+        namespace: null
+        db_path: ${env.SQLITE_STORE_DIR:=~/.llama/distributions/ollama}/huggingface_datasetio.db
   datasetio:
   - provider_id: huggingface
     provider_type: remote::huggingface
     config:
-      kvstore:
+      metadata_store:
         type: sqlite
         namespace: null
-        db_path: ${env.SQLITE_STORE_DIR:~/.llama/distributions/ollama}/huggingface_datasetio.db
+        db_path: ${env.SQLITE_STORE_DIR:=~/.llama/distributions/ollama}/huggingface_datasetio.db
   - provider_id: localfs
     provider_type: inline::localfs
     config:
-      kvstore:
+      metadata_store:
         type: sqlite
         namespace: null
-        db_path: ${env.SQLITE_STORE_DIR:~/.llama/distributions/ollama}/localfs_datasetio.db
+        db_path: ${env.SQLITE_STORE_DIR:=~/.llama/distributions/ollama}/localfs_datasetio.db
+  scoring:
+  - provider_id: basic
+    provider_type: inline::basic
+    config: {}
   tool_runtime:
   - provider_id: brave-search
     provider_type: remote::brave-search
     config:
-      api_key: ${env.BRAVE_SEARCH_API_KEY:}
+      api_key: ${env.BRAVE_SEARCH_API_KEY:+}
       max_results: 3
   - provider_id: tavily-search
     provider_type: remote::tavily-search
     config:
-      api_key: ${env.TAVILY_SEARCH_API_KEY:}
+      api_key: ${env.TAVILY_SEARCH_API_KEY:+}
       max_results: 3
   - provider_id: rag-runtime
     provider_type: inline::rag-runtime
     config: {}
-  - provider_id: model-context-protocol
-    provider_type: remote::model-context-protocol
-    config: {}
   - provider_id: wolfram-alpha
     provider_type: remote::wolfram-alpha
     config:
-      api_key: ${env.WOLFRAM_ALPHA_API_KEY:}
+      api_key: ${env.WOLFRAM_ALPHA_API_KEY:+}
+
 metadata_store:
   type: sqlite
-  db_path: ${env.SQLITE_STORE_DIR:~/.llama/distributions/ollama}/registry.db
+  db_path: ${env.SQLITE_STORE_DIR:=~/.llama/distributions/ollama}/registry.db
 models:
 - metadata: {}
   model_id: ${env.INFERENCE_MODEL}

tests/unit/server/test_replace_env_vars.py

@@ -26,39 +26,44 @@ class TestReplaceEnvVars(unittest.TestCase):
         self.assertEqual(replace_env_vars("${env.TEST_VAR}"), "test_value")
 
     def test_default_value_when_not_set(self):
-        self.assertEqual(replace_env_vars("${env.NOT_SET:default}"), "default")
+        self.assertEqual(replace_env_vars("${env.NOT_SET:=default}"), "default")
 
     def test_default_value_when_set(self):
-        self.assertEqual(replace_env_vars("${env.TEST_VAR:default}"), "test_value")
+        self.assertEqual(replace_env_vars("${env.TEST_VAR:=default}"), "test_value")
 
     def test_default_value_when_empty(self):
-        self.assertEqual(replace_env_vars("${env.EMPTY_VAR:default}"), "default")
+        self.assertEqual(replace_env_vars("${env.EMPTY_VAR:=default}"), "default")
+
+    def test_empty_var_no_default(self):
+        self.assertEqual(replace_env_vars("${env.EMPTY_VAR_NO_DEFAULT:+}"), None)
 
     def test_conditional_value_when_set(self):
-        self.assertEqual(replace_env_vars("${env.TEST_VAR+conditional}"), "conditional")
+        self.assertEqual(replace_env_vars("${env.TEST_VAR:+conditional}"), "conditional")
 
     def test_conditional_value_when_not_set(self):
-        self.assertEqual(replace_env_vars("${env.NOT_SET+conditional}"), "")
+        self.assertEqual(replace_env_vars("${env.NOT_SET:+conditional}"), None)
 
     def test_conditional_value_when_empty(self):
-        self.assertEqual(replace_env_vars("${env.EMPTY_VAR+conditional}"), "")
+        self.assertEqual(replace_env_vars("${env.EMPTY_VAR:+conditional}"), None)
 
     def test_conditional_value_with_zero(self):
-        self.assertEqual(replace_env_vars("${env.ZERO_VAR+conditional}"), "conditional")
+        self.assertEqual(replace_env_vars("${env.ZERO_VAR:+conditional}"), "conditional")
 
     def test_mixed_syntax(self):
-        self.assertEqual(replace_env_vars("${env.TEST_VAR:default} and ${env.NOT_SET+conditional}"), "test_value and ")
         self.assertEqual(
-            replace_env_vars("${env.NOT_SET:default} and ${env.TEST_VAR+conditional}"), "default and conditional"
+            replace_env_vars("${env.TEST_VAR:=default} and ${env.NOT_SET:+conditional}"), "test_value and "
+        )
+        self.assertEqual(
+            replace_env_vars("${env.NOT_SET:=default} and ${env.TEST_VAR:+conditional}"), "default and conditional"
         )
 
     def test_nested_structures(self):
         data = {
-            "key1": "${env.TEST_VAR:default}",
-            "key2": ["${env.NOT_SET:default}", "${env.TEST_VAR+conditional}"],
-            "key3": {"nested": "${env.NOT_SET+conditional}"},
+            "key1": "${env.TEST_VAR:=default}",
+            "key2": ["${env.NOT_SET:=default}", "${env.TEST_VAR:+conditional}"],
+            "key3": {"nested": "${env.NOT_SET:+conditional}"},
         }
-        expected = {"key1": "test_value", "key2": ["default", "conditional"], "key3": {"nested": ""}}
+        expected = {"key1": "test_value", "key2": ["default", "conditional"], "key3": {"nested": None}}
         self.assertEqual(replace_env_vars(data), expected)