Add model availability NUX metadata (#12972)

- replace show_nux with structured availability_nux model metadata
- expose availability NUX data through the app-server model API
- update shared fixtures and tests for the new field

codex-rs/app-server/tests/common/models_cache.rs

@@ -34,6 +34,7 @@ fn preset_to_info(preset: &ModelPreset, priority: i32) -> ModelInfo {
         default_reasoning_summary: ReasoningSummary::Auto,
         support_verbosity: false,
         default_verbosity: None,
+        availability_nux: None,
         apply_patch_tool_type: None,
         truncation_policy: TruncationPolicyConfig::bytes(10_000),
         supports_parallel_tool_calls: false,