feat: load config class when doing variable substitution

When using bash style substitution env variable in distribution
template, we are processing the string and convert it to the type
associated with the provider's config class. This allows us to return
the proper type. This is crucial for api key since they are not strings
anymore but SecretStr. If the key is unset we will get an empty string
which will result in a Pydantic error like:

```
ERROR    2025-09-25 21:40:44,565 __main__:527 core::server: Error creating app: 1 validation error for AnthropicConfig
         api_key
           Input should be a valid string
             For further information visit
             https://errors.pydantic.dev/2.11/v/string_type
```

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

tests/unit/server/test_replace_env_vars.py

@@ -8,7 +8,10 @@ import os
 
 import pytest
 
-from llama_stack.core.stack import replace_env_vars
+
+# Wrapper for backward compatibility in tests
+def replace_env_vars_compat(config, path=""):
+    return replace_env_vars_compat(config, path, None, None)
 
 
 @pytest.fixture
@@ -32,52 +35,54 @@ def setup_env_vars():
 
 
 def test_simple_replacement(setup_env_vars):
-    assert replace_env_vars("${env.TEST_VAR}") == "test_value"
+    assert replace_env_vars_compat("${env.TEST_VAR}") == "test_value"
 
 
 def test_default_value_when_not_set(setup_env_vars):
-    assert replace_env_vars("${env.NOT_SET:=default}") == "default"
+    assert replace_env_vars_compat("${env.NOT_SET:=default}") == "default"
 
 
 def test_default_value_when_set(setup_env_vars):
-    assert replace_env_vars("${env.TEST_VAR:=default}") == "test_value"
+    assert replace_env_vars_compat("${env.TEST_VAR:=default}") == "test_value"
 
 
 def test_default_value_when_empty(setup_env_vars):
-    assert replace_env_vars("${env.EMPTY_VAR:=default}") == "default"
+    assert replace_env_vars_compat("${env.EMPTY_VAR:=default}") == "default"
 
 
 def test_none_value_when_empty(setup_env_vars):
-    assert replace_env_vars("${env.EMPTY_VAR:=}") is None
+    assert replace_env_vars_compat("${env.EMPTY_VAR:=}") is None
 
 
 def test_value_when_set(setup_env_vars):
-    assert replace_env_vars("${env.TEST_VAR:=}") == "test_value"
+    assert replace_env_vars_compat("${env.TEST_VAR:=}") == "test_value"
 
 
 def test_empty_var_no_default(setup_env_vars):
-    assert replace_env_vars("${env.EMPTY_VAR_NO_DEFAULT:+}") is None
+    assert replace_env_vars_compat("${env.EMPTY_VAR_NO_DEFAULT:+}") is None
 
 
 def test_conditional_value_when_set(setup_env_vars):
-    assert replace_env_vars("${env.TEST_VAR:+conditional}") == "conditional"
+    assert replace_env_vars_compat("${env.TEST_VAR:+conditional}") == "conditional"
 
 
 def test_conditional_value_when_not_set(setup_env_vars):
-    assert replace_env_vars("${env.NOT_SET:+conditional}") is None
+    assert replace_env_vars_compat("${env.NOT_SET:+conditional}") is None
 
 
 def test_conditional_value_when_empty(setup_env_vars):
-    assert replace_env_vars("${env.EMPTY_VAR:+conditional}") is None
+    assert replace_env_vars_compat("${env.EMPTY_VAR:+conditional}") is None
 
 
 def test_conditional_value_with_zero(setup_env_vars):
-    assert replace_env_vars("${env.ZERO_VAR:+conditional}") == "conditional"
+    assert replace_env_vars_compat("${env.ZERO_VAR:+conditional}") == "conditional"
 
 
 def test_mixed_syntax(setup_env_vars):
-    assert replace_env_vars("${env.TEST_VAR:=default} and ${env.NOT_SET:+conditional}") == "test_value and "
-    assert replace_env_vars("${env.NOT_SET:=default} and ${env.TEST_VAR:+conditional}") == "default and conditional"
+    assert replace_env_vars_compat("${env.TEST_VAR:=default} and ${env.NOT_SET:+conditional}") == "test_value and "
+    assert (
+        replace_env_vars_compat("${env.NOT_SET:=default} and ${env.TEST_VAR:+conditional}") == "default and conditional"
+    )
 
 
 def test_nested_structures(setup_env_vars):
@@ -87,11 +92,11 @@ def test_nested_structures(setup_env_vars):
         "key3": {"nested": "${env.NOT_SET:+conditional}"},
     }
     expected = {"key1": "test_value", "key2": ["default", "conditional"], "key3": {"nested": None}}
-    assert replace_env_vars(data) == expected
+    assert replace_env_vars_compat(data) == expected
 
 
 def test_explicit_strings_preserved(setup_env_vars):
     # Explicit strings that look like numbers/booleans should remain strings
     data = {"port": "8080", "enabled": "true", "count": "123", "ratio": "3.14"}
     expected = {"port": "8080", "enabled": "true", "count": "123", "ratio": "3.14"}
-    assert replace_env_vars(data) == expected
+    assert replace_env_vars_compat(data) == expected