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/sentence_transformers/__init__.py

@@ -6,12 +6,14 @@
 
 from typing import Any
 
+from llama_stack.providers.datatypes import ProviderContext
 from llama_stack.providers.inline.inference.sentence_transformers.config import (
     SentenceTransformersInferenceConfig,
 )
 
 
 async def get_provider_impl(
+    context: ProviderContext,
     config: SentenceTransformersInferenceConfig,
     _deps: dict[str, Any],
 ):