Add a skill invocation extension contributor (#32200)

## What changed

- Add `SkillInvocationContributor` and registration support to the extension API.
- Provide invocation callbacks with session-, thread-, and turn-scoped extension data, the turn ID, skill resource, and explicit or implicit invocation kind.
- Notify registered contributors when Codex observes a deduplicated implicit skill invocation.

## Testing

- Extend the extension registry round-trip test to cover skill invocation contributors.

GitOrigin-RevId: f428336728889d9bf4fba41a7dd8d3a6f9b9728c

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

@@ -42,6 +42,9 @@ pub use contributors::PreviousWorldStateSection;
 pub use contributors::PromptFragment;
 pub use contributors::PromptSlot;
 pub use contributors::RenderedWorldStateFragment;
+pub use contributors::SkillInvocationContributor;
+pub use contributors::SkillInvocationInput;
+pub use contributors::SkillInvocationKind;
 pub use contributors::ThreadIdleInput;
 pub use contributors::ThreadLifecycleContributor;
 pub use contributors::ThreadResumeInput;