- _on_graph_activate: Clear stale credential references for optional
fields instead of blocking the save. Checks both node metadata
(credentials_optional) and block schema (field not in required_fields).
- _validate_node_input_credentials: Use block schema's required_fields
as fallback for credentials_optional check, so MCP blocks with
default={} credentials are properly treated as optional.
- Set credentials_optional metadata on new MCP nodes in the frontend.