[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/client.rs

@@ -575,6 +575,8 @@ async fn process_sse<S>(
             }
             "response.content_part.done"
             | "response.function_call_arguments.delta"
+            | "response.custom_tool_call_input.delta"
+            | "response.custom_tool_call_input.done" // also emitted as response.output_item.done
             | "response.in_progress"
             | "response.output_item.added"
             | "response.output_text.done" => {