fix: Use re-entrancy and concurrency safe context managers for provider data (#1498)

Concurrent requests should not trample (or reuse) each others' provider
data. Provider data should be scoped to each request.

## Test Plan

Set the uvicorn server to have a single worker process + thread by
updating the config:
```python
    uvicorn_config = {
        ...
        "workers": 1,
        "loop": "asyncio",
    }
```

Then perform the following steps on `origin/main` (without this change).

(1) Run the server using `llama stack run dev` without having
`FIREWORKS_API_KEY` in the environment.

(2) Run a test by specifying the FIREWORKS_API_KEY env var so it gets
stored in the thread local
```
pytest -s -v tests/integration/inference/test_text_inference.py \
    --stack-config http://localhost:8321 \
    --text-model accounts/fireworks/models/llama-v3p1-8b-instruct \
    -k test_text_chat_completion_with_tool_calling_and_streaming \
     --env FIREWORKS_API_KEY=<...>
``` 
Ensure you don't have any other API keys in the environment (otherwise
the bug will not reproduce due to other specifics in our testing code.)
Verify this works.

(3) Run the same command again without specifying FIREWORKS_API_KEY. See
that the request actually succeeds when it *should have failed*.


----
Now do the same tests on this branch, verify step (3) results in
failure.

Finally, run the full `test_text_inference.py` test suite with this
change, verify it succeeds.

llama_stack/distribution/request_headers.py

@@ -4,16 +4,62 @@
 # This source code is licensed under the terms described in the LICENSE file in
 # the root directory of this source tree.
 
+import contextvars
 import json
 import logging
-import threading
-from typing import Any, Dict
+from typing import Any, AsyncGenerator, ContextManager, Dict, Optional, TypeVar
 
 from .utils.dynamic import instantiate_class_type
 
 log = logging.getLogger(__name__)
 
-_THREAD_LOCAL = threading.local()
+# Context variable for request provider data
+_provider_data_var = contextvars.ContextVar("provider_data", default=None)
+
+
+class RequestProviderDataContext(ContextManager):
+    """Context manager for request provider data"""
+
+    def __init__(self, provider_data: Optional[Dict[str, Any]] = None):
+        self.provider_data = provider_data
+        self.token = None
+
+    def __enter__(self):
+        # Save the current value and set the new one
+        self.token = _provider_data_var.set(self.provider_data)
+        return self
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        # Restore the previous value
+        if self.token is not None:
+            _provider_data_var.reset(self.token)
+
+
+T = TypeVar("T")
+
+
+def preserve_headers_context_async_generator(gen: AsyncGenerator[T, None]) -> AsyncGenerator[T, None]:
+    """
+    Wraps an async generator to preserve request headers context variables across iterations.
+
+    This ensures that context variables set during generator creation are
+    available during each iteration of the generator, even if the original
+    context manager has exited.
+    """
+    # Capture the current context value right now
+    context_value = _provider_data_var.get()
+
+    async def wrapper():
+        while True:
+            # Set context before each anext() call
+            _ = _provider_data_var.set(context_value)
+            try:
+                item = await gen.__anext__()
+                yield item
+            except StopAsyncIteration:
+                break
+
+    return wrapper()
 
 
 class NeedsRequestProviderData:
@@ -26,7 +72,7 @@ class NeedsRequestProviderData:
         if not validator_class:
             raise ValueError(f"Provider {provider_type} does not have a validator")
 
-        val = getattr(_THREAD_LOCAL, "provider_data_header_value", None)
+        val = _provider_data_var.get()
         if not val:
             return None
 
@@ -36,25 +82,32 @@ class NeedsRequestProviderData:
             return provider_data
         except Exception as e:
             log.error(f"Error parsing provider data: {e}")
+            return None
 
 
-def set_request_provider_data(headers: Dict[str, str]):
+def parse_request_provider_data(headers: Dict[str, str]) -> Optional[Dict[str, Any]]:
+    """Parse provider data from request headers"""
     keys = [
         "X-LlamaStack-Provider-Data",
         "x-llamastack-provider-data",
     ]
+    val = None
     for key in keys:
         val = headers.get(key, None)
         if val:
             break
 
     if not val:
-        return
+        return None
 
     try:
-        val = json.loads(val)
+        return json.loads(val)
     except json.JSONDecodeError:
-        log.error("Provider data not encoded as a JSON object!", val)
-        return
+        log.error("Provider data not encoded as a JSON object!")
+        return None
 
-    _THREAD_LOCAL.provider_data_header_value = val
+
+def request_provider_data_context(headers: Dict[str, str]) -> ContextManager:
+    """Context manager that sets request provider data from headers for the duration of the context"""
+    provider_data = parse_request_provider_data(headers)
+    return RequestProviderDataContext(provider_data)