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/inline/agents/meta_reference/config.py

@@ -13,19 +13,26 @@ from llama_stack.providers.utils.kvstore.config import SqliteKVStoreConfig
 from llama_stack.providers.utils.sqlstore.sqlstore import SqliteSqlStoreConfig, SqlStoreConfig
 
 
+class AgentPersistenceConfig(BaseModel):
+    """Nested persistence configuration for agents."""
+    agent_state: KVStoreConfig
+    responses: SqlStoreConfig
+
+
 class MetaReferenceAgentsImplConfig(BaseModel):
-    persistence_store: KVStoreConfig
-    responses_store: SqlStoreConfig
+    persistence: AgentPersistenceConfig
 
     @classmethod
     def sample_run_config(cls, __distro_dir__: str) -> dict[str, Any]:
         return {
-            "persistence_store": SqliteKVStoreConfig.sample_run_config(
-                __distro_dir__=__distro_dir__,
-                db_name="agents_store.db",
-            ),
-            "responses_store": SqliteSqlStoreConfig.sample_run_config(
-                __distro_dir__=__distro_dir__,
-                db_name="responses_store.db",
-            ),
+            "persistence": {
+                "agent_state": SqliteKVStoreConfig.sample_run_config(
+                    __distro_dir__=__distro_dir__,
+                    db_name="agents_store.db",
+                ),
+                "responses": SqliteSqlStoreConfig.sample_run_config(
+                    __distro_dir__=__distro_dir__,
+                    db_name="responses_store.db",
+                ),
+            }
         }