Add model availability NUX tooltips (#13021)

- override startup tooltips with model availability NUX and persist
per-model show counts in config
- stop showing each model after four exposures and fall back to normal
tooltips

codex-rs/tui/tests/suite/mod.rs

@@ -1,4 +1,5 @@
 // Aggregates all former standalone integration tests as modules.
+mod model_availability_nux;
 mod no_panic_on_startup;
 mod status_indicator;
 mod vt100_history;