Attribute executor skill invocations to plugins (#39309)

## What changed

- Carry selected plugin identities from MCP discovery into per-turn extension data, limited to capability roots that are ready for the turn.
- Annotate executor skill catalog entries with the matching plugin ID and `user` scope.
- Include that attribution in `skill_invocation` analytics for both explicit and implicit executor skill invocations.

## Testing

- Extend the executor skills app-server test to verify plugin ID, skill scope, and invocation type for explicit and implicit analytics events.

GitOrigin-RevId: 9345600ae29bcb8eb894fffd2d6f7b05f22ebf74

codex-rs/ext/extension-api/src/lib.rs

@@ -46,6 +46,8 @@ pub use contributors::PreviousWorldStateSection;
 pub use contributors::PromptFragment;
 pub use contributors::PromptSlot;
 pub use contributors::RenderedWorldStateFragment;
+pub use contributors::SelectedPluginIdentity;
+pub use contributors::SelectedPluginSnapshot;
 pub use contributors::SkillInvocationContributor;
 pub use contributors::SkillInvocationInput;
 pub use contributors::SkillInvocationKind;