Update exception handling for valkey-glide compatibility

## Objective

Replace redis-py exception handling with valkey-glide compatible error handling.

## Files to modify

- `src/ai/backend/manager/models/health.py` (line 9)
- `src/ai/backend/manager/models/session.py`
- Line 27: Import statements
- Lines 1784, 1797, 1832: Exception handling
- Specific case: `redis.exceptions.ChildDeadlockedError`
- `src/ai/backend/common/clients/valkey_client/client.py` (line 19)
- Sentinel compatibility layer

## Tasks

- [ ] Identify valkey-glide equivalent exceptions
- [ ] Replace `redis.exceptions` imports
- [ ] Update exception catching blocks
- [ ] Ensure error handling semantics remain correct
- [ ] Handle cases like ChildDeadlockedError, ConnectionError, TimeoutError

## Related

Part of BA-2585 - Retire redis-py entirely

JIRA Issue: BA-3272