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/vector_io/milvus/__init__.py

@@ -6,14 +6,15 @@
 
 from typing import Any
 
-from llama_stack.providers.datatypes import Api
+from llama_stack.providers.datatypes import Api, ProviderContext
 
 from .config import MilvusVectorIOConfig
 
 
-async def get_provider_impl(config: MilvusVectorIOConfig, deps: dict[Api, Any]):
+async def get_provider_impl(context: ProviderContext, config: MilvusVectorIOConfig, deps: dict[Api, Any]):
     from llama_stack.providers.remote.vector_io.milvus.milvus import MilvusVectorIOAdapter
 
+    # Pass config directly since MilvusVectorIOAdapter doesn't accept context
     impl = MilvusVectorIOAdapter(config, deps[Api.inference])
     await impl.initialize()
     return impl