Frontend: Include credentials field in MCP block's dynamic input schema
so users can select OAuth credentials from the node form. Separate
credentials from tool_arguments in FormCreator to store them at the
correct level in hardcodedValues.
Backend: Add _auto_lookup_credential fallback in MCPToolBlock.run() for
legacy nodes that don't have credentials explicitly set. This resolves
the credential by matching mcp_server_url in stored OAuth metadata.