refactor: Add ProviderContext for a flexible storage directory

- Introduce ProviderContext class to decouple provider storage paths from absolute paths
- Add storage_dir attribute to StackRunConfig to accept CLI options
- Implement storage directory resolution with prioritized fallbacks:
  1. CLI option (--state-directory)
  2. Environment variable (LLAMA_STACK_STATE_DIR)
  3. Default distribution directory
- Standardize provider signatures to follow context, config, deps pattern
- Update provider implementations to use the new context-based approach
- Add comprehensive tests to verify state directory resolution

tests/unit/providers/inference/test_remote_vllm.py

@@ -62,9 +62,13 @@ class MockInferenceAdapterWithSleep:
             # ruff: noqa: N802
             def do_POST(self):
                 time.sleep(sleep_time)
+                response_json = json.dumps(response).encode("utf-8")
                 self.send_response(code=200)
+                self.send_header("Content-Type", "application/json")
+                self.send_header("Content-Length", str(len(response_json)))
                 self.end_headers()
-                self.wfile.write(json.dumps(response).encode("utf-8"))
+                self.wfile.write(response_json)
+                self.wfile.flush()
 
         self.request_handler = DelayedRequestHandler