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/eval/meta_reference/__init__.py

@@ -6,11 +6,13 @@
 from typing import Any
 
 from llama_stack.distribution.datatypes import Api
+from llama_stack.providers.datatypes import ProviderContext
 
 from .config import MetaReferenceEvalConfig
 
 
 async def get_provider_impl(
+    context: ProviderContext,
     config: MetaReferenceEvalConfig,
     deps: dict[Api, Any],
 ):