feat!: providers use unified 'persistence' field

BREAKING CHANGE: Provider config field names changed for semantic clarity

- Rename kvstore → persistence for all providers
- Simple providers: flat persistence with backend reference
- Complex providers (agents): nested persistence.agent_state + persistence.responses
- Files provider: metadata_store → persistence
- Provider configs now clearly express 'how do I persist?' not 'what type of store?'

Example:
  # Before
  config:
    kvstore:
      backend: kvstore
      namespace: faiss

  # After
  config:
    persistence:
      backend: kvstore
      namespace: faiss

  # Agents (nested)
  config:
    persistence:
      agent_state:
        backend: kvstore
        namespace: agents
      responses:
        backend: sqlstore
        namespace: responses

llama_stack/providers/remote/vector_io/chroma/chroma.py

@@ -150,7 +150,7 @@ class ChromaVectorIOAdapter(OpenAIVectorStoreMixin, VectorIO, VectorDBsProtocolP
         self.files_api = files_api
 
     async def initialize(self) -> None:
-        self.kvstore = await kvstore_impl(self.config.kvstore)
+        self.kvstore = await kvstore_impl(self.config.persistence)
         self.vector_db_store = self.kvstore
 
         if isinstance(self.config, RemoteChromaVectorIOConfig):

llama_stack/providers/remote/vector_io/chroma/config.py

@@ -15,13 +15,13 @@ from llama_stack.schema_utils import json_schema_type
 @json_schema_type
 class ChromaVectorIOConfig(BaseModel):
     url: str | None
-    kvstore: KVStoreConfig = Field(description="Config for KV store backend")
+    persistence: KVStoreConfig = Field(description="Config for KV store backend")
 
     @classmethod
     def sample_run_config(cls, __distro_dir__: str, url: str = "${env.CHROMADB_URL}", **kwargs: Any) -> dict[str, Any]:
         return {
             "url": url,
-            "kvstore": SqliteKVStoreConfig.sample_run_config(
+            "persistence": SqliteKVStoreConfig.sample_run_config(
                 __distro_dir__=__distro_dir__,
                 db_name="chroma_remote_registry.db",
             ),