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/milvus/config.py

@@ -17,7 +17,7 @@ class MilvusVectorIOConfig(BaseModel):
     uri: str = Field(description="The URI of the Milvus server")
     token: str | None = Field(description="The token of the Milvus server")
     consistency_level: str = Field(description="The consistency level of the Milvus server", default="Strong")
-    kvstore: KVStoreConfig = Field(description="Config for KV store backend")
+    persistence: KVStoreConfig = Field(description="Config for KV store backend")
 
     # This configuration allows additional fields to be passed through to the underlying Milvus client.
     # See the [Milvus](https://milvus.io/docs/install-overview.md) documentation for more details about Milvus in general.
@@ -28,7 +28,7 @@ class MilvusVectorIOConfig(BaseModel):
         return {
             "uri": "${env.MILVUS_ENDPOINT}",
             "token": "${env.MILVUS_TOKEN}",
-            "kvstore": SqliteKVStoreConfig.sample_run_config(
+            "persistence": SqliteKVStoreConfig.sample_run_config(
                 __distro_dir__=__distro_dir__,
                 db_name="milvus_remote_registry.db",
             ),

llama_stack/providers/remote/vector_io/milvus/milvus.py

@@ -320,7 +320,7 @@ class MilvusVectorIOAdapter(OpenAIVectorStoreMixin, VectorIO, VectorDBsProtocolP
         self.metadata_collection_name = "openai_vector_stores_metadata"
 
     async def initialize(self) -> None:
-        self.kvstore = await kvstore_impl(self.config.kvstore)
+        self.kvstore = await kvstore_impl(self.config.persistence)
         start_key = VECTOR_DBS_PREFIX
         end_key = f"{VECTOR_DBS_PREFIX}\xff"
         stored_vector_dbs = await self.kvstore.values_in_range(start_key, end_key)