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/files/s3/config.py

@@ -24,7 +24,7 @@ class S3FilesImplConfig(BaseModel):
     auto_create_bucket: bool = Field(
         default=False, description="Automatically create the S3 bucket if it doesn't exist"
     )
-    metadata_store: SqlStoreConfig = Field(description="SQL store configuration for file metadata")
+    persistence: SqlStoreConfig = Field(description="SQL store configuration for file metadata")
 
     @classmethod
     def sample_run_config(cls, __distro_dir__: str) -> dict[str, Any]:
@@ -35,7 +35,7 @@ class S3FilesImplConfig(BaseModel):
             "aws_secret_access_key": "${env.AWS_SECRET_ACCESS_KEY:=}",
             "endpoint_url": "${env.S3_ENDPOINT_URL:=}",
             "auto_create_bucket": "${env.S3_AUTO_CREATE_BUCKET:=false}",
-            "metadata_store": SqliteSqlStoreConfig.sample_run_config(
+            "persistence": SqliteSqlStoreConfig.sample_run_config(
                 __distro_dir__=__distro_dir__,
                 db_name="s3_files_metadata.db",
             ),