feat: added SetDefaultModel to JSON-RPC server (#3512)

This adds `SetDefaultModel`, which takes `model` and `reasoning_effort`
as optional fields. If set, the field will overwrite what is in the
user's `config.toml`.

This reuses logic that was added to support the `/model` command in the
TUI: https://github.com/openai/codex/pull/2799.

codex-rs/mcp-server/tests/suite/mod.rs

@@ -9,5 +9,6 @@ mod interrupt;
 mod list_resume;
 mod login;
 mod send_message;
+mod set_default_model;
 mod user_agent;
 mod user_info;