fix(cliproxy): delegate OpenAI-compatible OAuth refresh to plugin auth providers

- Added a plugin refresh-compat executor wrapper that forwards normal OpenAI-compat execution paths while routing `Refresh` to plugin `AuthProvider`/Home refresh logic.
- Updated refresh lookup to use the effective executor key from auth metadata so namespaced compatibility providers can resolve their refresh executors correctly.
- Changed OpenAI-compat registration to wrap built-in executors with the plugin-refresh wrapper when a matching plugin auth provider exists, while preserving bare executors otherwise.
- Made `OpenAICompatExecutor.Refresh` fail fast for OAuth-style credentials (with refresh tokens) instead of silently returning unchanged auth.

Closes: #4719