## What changed
- Add the originating Responses item ID to MCP tool request metadata as
`_meta.itemId`.
- Preserve the initiating `exec` item ID across code-mode cells so nested MCP
calls carry the same origin.
## Testing
- Cover metadata generation for direct and code-mode MCP tool calls.
GitOrigin-RevId: 1161cac64f1cc3a6d4dac4368e63804bd9d2c417