Include originating item IDs in MCP request metadata (#40866)

## 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