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

llama_stack/providers/remote/inference/vllm/__init__.py

@@ -7,7 +7,7 @@
 from .config import VLLMInferenceAdapterConfig
 
 
-async def get_adapter_impl(config: VLLMInferenceAdapterConfig, _deps):
+async def get_adapter_impl(config: VLLMInferenceAdapterConfig, deps):
     from .vllm import VLLMInferenceAdapter
 
     assert isinstance(config, VLLMInferenceAdapterConfig), f"Unexpected config type: {type(config)}"