refactor(test): introduce --stack-config and simplify options (#1404)

You now run the integration tests with these options:

```bash
Custom options:
  --stack-config=STACK_CONFIG
                        a 'pointer' to the stack. this can be either be:
                        (a) a template name like `fireworks`, or
                        (b) a path to a run.yaml file, or
                        (c) an adhoc config spec, e.g.
                        `inference=fireworks,safety=llama-guard,agents=meta-
                        reference`
  --env=ENV             Set environment variables, e.g. --env KEY=value
  --text-model=TEXT_MODEL
                        comma-separated list of text models. Fixture name:
                        text_model_id
  --vision-model=VISION_MODEL
                        comma-separated list of vision models. Fixture name:
                        vision_model_id
  --embedding-model=EMBEDDING_MODEL
                        comma-separated list of embedding models. Fixture name:
                        embedding_model_id
  --safety-shield=SAFETY_SHIELD
                        comma-separated list of safety shields. Fixture name:
                        shield_id
  --judge-model=JUDGE_MODEL
                        comma-separated list of judge models. Fixture name:
                        judge_model_id
  --embedding-dimension=EMBEDDING_DIMENSION
                        Output dimensionality of the embedding model to use for
                        testing. Default: 384
  --record-responses    Record new API responses instead of using cached ones.
  --report=REPORT       Path where the test report should be written, e.g.
                        --report=/path/to/report.md

```

Importantly, if you don't specify any of the models (text-model,
vision-model, etc.) the relevant tests will get **skipped!**

This will make running tests somewhat more annoying since all options
will need to be specified. We will make this easier by adding some easy
wrapper yaml configs.

## Test Plan

Example:

```bash
ashwin@ashwin-mbp ~/local/llama-stack/tests/integration (unify_tests) $ 
LLAMA_STACK_CONFIG=fireworks pytest -s -v inference/test_text_inference.py \
   --text-model meta-llama/Llama-3.2-3B-Instruct 
```

llama_stack/distribution/stack.py

@@ -7,6 +7,7 @@
 import importlib.resources
 import os
 import re
+import tempfile
 from typing import Any, Dict, Optional
 
 import yaml
@@ -33,10 +34,11 @@ from llama_stack.apis.telemetry import Telemetry
 from llama_stack.apis.tools import RAGToolRuntime, ToolGroups, ToolRuntime
 from llama_stack.apis.vector_dbs import VectorDBs
 from llama_stack.apis.vector_io import VectorIO
-from llama_stack.distribution.datatypes import StackRunConfig
+from llama_stack.distribution.datatypes import Provider, StackRunConfig
 from llama_stack.distribution.distribution import get_provider_registry
 from llama_stack.distribution.resolver import ProviderRegistry, resolve_impls
 from llama_stack.distribution.store.registry import create_dist_registry
+from llama_stack.distribution.utils.dynamic import instantiate_class_type
 from llama_stack.providers.datatypes import Api
 
 
@@ -228,3 +230,53 @@ def get_stack_run_config_from_template(template: str) -> StackRunConfig:
         run_config = yaml.safe_load(path.open())
 
     return StackRunConfig(**replace_env_vars(run_config))
+
+
+def run_config_from_adhoc_config_spec(
+    adhoc_config_spec: str, provider_registry: Optional[ProviderRegistry] = None
+) -> StackRunConfig:
+    """
+    Create an adhoc distribution from a list of API providers.
+
+    The list should be of the form "api=provider", e.g. "inference=fireworks". If you have
+    multiple pairs, separate them with commas or semicolons, e.g. "inference=fireworks,safety=llama-guard,agents=meta-reference"
+    """
+
+    api_providers = adhoc_config_spec.replace(";", ",").split(",")
+    provider_registry = provider_registry or get_provider_registry()
+
+    distro_dir = tempfile.mkdtemp()
+    provider_configs_by_api = {}
+    for api_provider in api_providers:
+        api_str, provider = api_provider.split("=")
+        api = Api(api_str)
+
+        providers_by_type = provider_registry[api]
+        provider_spec = providers_by_type.get(provider)
+        if not provider_spec:
+            provider_spec = providers_by_type.get(f"inline::{provider}")
+        if not provider_spec:
+            provider_spec = providers_by_type.get(f"remote::{provider}")
+
+        if not provider_spec:
+            raise ValueError(
+                f"Provider {provider} (or remote::{provider} or inline::{provider}) not found for API {api}"
+            )
+
+        # call method "sample_run_config" on the provider spec config class
+        provider_config_type = instantiate_class_type(provider_spec.config_class)
+        provider_config = replace_env_vars(provider_config_type.sample_run_config(__distro_dir__=distro_dir))
+
+        provider_configs_by_api[api_str] = [
+            Provider(
+                provider_id=provider,
+                provider_type=provider_spec.provider_type,
+                config=provider_config,
+            )
+        ]
+    config = StackRunConfig(
+        image_name="distro-test",
+        apis=list(provider_configs_by_api.keys()),
+        providers=provider_configs_by_api,
+    )
+    return config