feat(codex): add Alpha Search API key support with configurable endpoint

- Introduced `alpha-search` flag for Codex API keys to enable Alpha Search functionality.
- Added `CredentialPolicyCodexAlphaSearchV1` for filtering and validating API keys compatible with Alpha Search.
- Extended configuration, synthesizer, and watcher functionality to track and manage `alpha-search` flag changes.
- Updated Codex executor to dynamically construct Alpha Search endpoint based on API key attributes.
- Enhanced tests to validate Alpha Search flag handling, endpoint construction, and credential filtering.

Closes: #4692