issue: #46762
Copy the fieldIndexes map while holding the read lock to prevent data
race. The original code released the lock before iterating over the map,
which could cause concurrent access issues.
Affected methods:
- GetSegmentIndexState
- GetIndexedSegments
- IsUnIndexedSegment
- GetSegmentIndexedFields
Signed-off-by: Wei Liu <wei.liu@zilliz.com>