fix: preserve search aggregation group-by metadata (#50176)

issue: #49840

## Summary
- Preserve plural group-by field IDs when shallow-copying search
requests for query nodes.
- Add regression coverage for SearchRequest shallow-copy metadata
propagation.
- Re-enable the growing-segment search aggregation E2E case.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Signed-off-by: MrPresent-Han <chun.han@gmail.com>
Co-authored-by: MrPresent-Han <chun.han@gmail.com>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>