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/codex-api/tests/models_integration.rs

@@ -82,6 +82,7 @@ async fn models_client_hits_models_endpoint() {
             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,