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

tests/unit/test_state_dir_resolution.py

@@ -0,0 +1,43 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+from pathlib import Path
+
+from llama_stack.distribution.resolver import resolve_storage_dir
+
+
+class DummyConfig:
+    pass
+
+
+def test_storage_dir_cli(monkeypatch):
+    config = DummyConfig()
+    config.storage_dir = "/cli/dir"
+    monkeypatch.delenv("LLAMA_STACK_STORAGE_DIR", raising=False)
+    result = resolve_storage_dir(config, "distro")
+    assert result == Path("/cli/dir")
+
+
+def test_storage_dir_env(monkeypatch):
+    config = DummyConfig()
+    if hasattr(config, "storage_dir"):
+        delattr(config, "storage_dir")
+    monkeypatch.setenv("LLAMA_STACK_STORAGE_DIR", "/env/dir")
+    result = resolve_storage_dir(config, "distro")
+    assert result == Path("/env/dir")
+
+
+def test_storage_dir_fallback(monkeypatch):
+    # Mock the DISTRIBS_BASE_DIR
+    monkeypatch.setattr("llama_stack.distribution.utils.config_dirs.DISTRIBS_BASE_DIR", Path("/mock/distribs"))
+
+    config = DummyConfig()
+    if hasattr(config, "storage_dir"):
+        delattr(config, "storage_dir")
+    monkeypatch.delenv("LLAMA_STACK_STORAGE_DIR", raising=False)
+
+    result = resolve_storage_dir(config, "distro")
+    assert result == Path("/mock/distribs/distro")