Expose app-server diagnostics through the experimental API (#37470)

## What changed

- Add the experimental `server/diagnostics` request, returning the app-server process ID, resident memory, platform-dependent physical footprint, and registered diagnostic gauges.
- Export the new protocol types in the generated JSON and TypeScript schemas and document the request.

## Testing

- Verify the response includes process measurements and the registered live-thread gauge.
- Verify the request is rejected unless `capabilities.experimentalApi` is enabled.

GitOrigin-RevId: 099d60a9c421a74ff1a72fe524e0dfeb89cf1057

codex-rs/app-server-protocol/src/protocol/v2/mod.rs

@@ -7,6 +7,7 @@ mod collaboration_mode;
 mod command_exec;
 mod config;
 mod current_time;
+mod diagnostics;
 mod environment;
 mod experimental_feature;
 mod feedback;
@@ -35,6 +36,7 @@ pub use collaboration_mode::*;
 pub use command_exec::*;
 pub use config::*;
 pub use current_time::*;
+pub use diagnostics::*;
 pub use environment::*;
 pub use experimental_feature::*;
 pub use feedback::*;