feat(models): list models available via provider_data header (#3968)

## Summary

When users provide API keys via `X-LlamaStack-Provider-Data` header,
`models.list()` now returns models they can access from those providers,
not just pre-registered models from the registry.

This complements the routing fix from f88416ef8 which enabled inference
calls with `provider_id/model_id` format for unregistered models. Users
can now discover which models are available to them before making
inference requests.

The implementation reuses
`NeedsRequestProviderData.get_request_provider_data()` to validate
credentials, then dynamically fetches models from providers without
caching them since they're user-specific. Registry models take
precedence to respect any pre-configured aliases.

## Test Script

```python
#!/usr/bin/env python3
import json
import os
from openai import OpenAI

# Test 1: Without provider_data header
client = OpenAI(base_url="http://localhost:8321/v1/openai/v1", api_key="dummy")
models = client.models.list()
anthropic_without = [m.id for m in models.data if m.id and "anthropic" in m.id]
print(f"Without header: {len(models.data)} models, {len(anthropic_without)} anthropic")

# Test 2: With provider_data header containing Anthropic API key
anthropic_api_key = os.environ["ANTHROPIC_API_KEY"]
client_with_key = OpenAI(
    base_url="http://localhost:8321/v1/openai/v1",
    api_key="dummy",
    default_headers={
        "X-LlamaStack-Provider-Data": json.dumps({"anthropic_api_key": anthropic_api_key})
    }
)
models_with_key = client_with_key.models.list()
anthropic_with = [m.id for m in models_with_key.data if m.id and "anthropic" in m.id]
print(f"With header: {len(models_with_key.data)} models, {len(anthropic_with)} anthropic")
print(f"Anthropic models: {anthropic_with}")

assert len(anthropic_with) > len(anthropic_without), "Should have more anthropic models with API key"
print("\n✓ Test passed!")
```

Run with a stack that has Anthropic provider configured (but without API
key in config):
```bash
ANTHROPIC_API_KEY=sk-ant-... python test_provider_data_models.py
```

src/llama_stack/providers/remote/inference/anthropic/anthropic.py

@@ -33,4 +33,5 @@ class AnthropicInferenceAdapter(OpenAIMixin):
         return "https://api.anthropic.com/v1"
 
     async def list_provider_model_ids(self) -> Iterable[str]:
-        return [m.id async for m in AsyncAnthropic(api_key=self.get_api_key()).models.list()]
+        api_key = self._get_api_key_from_config_or_provider_data()
+        return [m.id async for m in AsyncAnthropic(api_key=api_key).models.list()]

src/llama_stack/providers/remote/inference/databricks/databricks.py

@@ -33,10 +33,11 @@ class DatabricksInferenceAdapter(OpenAIMixin):
 
     async def list_provider_model_ids(self) -> Iterable[str]:
         # Filter out None values from endpoint names
+        api_token = self._get_api_key_from_config_or_provider_data()
         return [
             endpoint.name  # type: ignore[misc]
             for endpoint in WorkspaceClient(
-                host=self.config.url, token=self.get_api_key()
+                host=self.config.url, token=api_token
             ).serving_endpoints.list()  # TODO: this is not async
         ]