feat(executor): add conditional support for Claude fast mode beta

- Introduced `claudeFastModeBeta` constant and logic to append it conditionally based on the `speed` attribute in the request body.
- Ensured `fast-mode-2026-02-01` beta inclusion only when explicitly requested or if the `speed` is set to `fast`.
- Updated `applyClaudeHeaders` to dynamically handle fast-mode beta and adjust headers accordingly.
- Enhanced beta extraction and addition with a new helper `appendClaudeFastModeBeta`.
- Improved flexibility in beta handling by preserving requested overrides while ensuring compatibility with default configurations.

Closes: #4703