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/inline/inference/vllm/__init__.py

@@ -6,12 +6,14 @@
 
 from typing import Any
 
+from llama_stack.providers.datatypes import ProviderContext
+
 from .config import VLLMConfig
 
 
-async def get_provider_impl(config: VLLMConfig, _deps: dict[str, Any]):
+async def get_provider_impl(context: ProviderContext, config: VLLMConfig, deps: dict[str, Any]):
     from .vllm import VLLMInferenceImpl
 
-    impl = VLLMInferenceImpl(config)
+    impl = VLLMInferenceImpl(context, config)
     await impl.initialize()
     return impl