issue: #43794
Fix the issue where L0 segments were not correctly selecting appropriate
delegators during loading, which could cause load failures or incorrect
delegator assignments.
Changes include:
- Add special handling for L0 segments in delegator selection logic
- Find delegators that are missing the L0 segment for direct loading
- Fallback to existing serviceable delegator selection when no suitable
delegator is found for L0 segments
- Add comprehensive test coverage for L0 segment loading scenarios
- Test delegator selection when some delegators are missing segments
- Test fallback behavior when all delegators already have the segment
- Test error handling when no delegators are available
Signed-off-by: Wei Liu <wei.liu@zilliz.com>