[apply_patch] freeform apply_patch tool (#2576)

## Summary
GPT-5 introduced the concept of [custom
tools](https://platform.openai.com/docs/guides/function-calling#custom-tools),
which allow the model to send a raw string result back, simplifying
json-escape issues. We are migrating gpt-5 to use this by default.

However, gpt-oss models do not support custom tools, only normal
functions. So we keep both tool definitions, and provide whichever one
the model family supports.

## Testing
- [x] Tested locally with various models
- [x] Unit tests pass

codex-rs/core/src/lib.rs

@@ -50,6 +50,7 @@ pub mod seatbelt;
 pub mod shell;
 pub mod spawn;
 pub mod terminal;
+mod tool_apply_patch;
 pub mod turn_diff_tracker;
 pub mod user_agent;
 mod user_notification;