fix: fall back when projectless generator asset is unavailable

[DamienZCollins]

Summary

• Treat the Codex App projectless-thread-* generator as an optional compatibility layer.
• When the generator asset is unavailable, cache that capability result for the active new-task revision and forward the native renderer or app-server request unchanged.
• Keep generated projectless overrides and genuine generator errors unchanged when the generator is available.
• Add regression coverage for both renderer and app-server fallback paths.

Validation

• node --check assets/inject/renderer-inject.js
• cargo fmt --check
• cargo test -p codex-plus-core --test cdp_bridge injection_script_applies_projectless_main_window_contract -- --exact
• cargo test -p codex-plus-core --test cdp_bridge injection_script_ (45 passed)

Fixes #1738

[DamienZCollins]

本地验证已完成：

• node --check assets/inject/renderer-inject.js
• cargo fmt --check
• cargo test -p codex-plus-core --test cdp_bridge injection_script_applies_projectless_main_window_contract -- --exact
• cargo test -p codex-plus-core --test cdp_bridge injection_script_ (45 passed)

GitHub 已创建 PR build artifacts run，但当前为 action_required。麻烦维护者批准/重新运行来自 fork 的三平台检查，谢谢。