### What problem does this PR solve?
Skip duplicate errors to avoid 'create_idx' failures caused by slow
metadata refresh or external modifications.
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
### What problem does this PR solve?
When there are multiple users, parsing a document for a new user can
trigger the reuse of column objects, leading to the error
`sqlalchemy.exc.ArgumentError: Column object 'id' already assigned to
Table xxx`.
### Type of change
- [x] Bug Fix (non-breaking change which fixes an issue)
### What problem does this PR solve?
Manage message and use in agent.
Issue #4213
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
### What problem does this PR solve?
Enhance OBConnection.search for better performance. Main changes:
1. Use string type of vector array in distance func for better parsing
performance.
2. Manually set max_connections as pool size instead of using default
value.
3. Set 'fulltext_search_columns' when starting.
4. Cache the results of the table existence check (we will never drop
the table).
5. Remove unused 'group_results' logic.
6. Add the `USE_FULLTEXT_FIRST_FUSION_SEARCH` flag, and the
corresponding fusion search SQL when it's false.
### Type of change
- [x] Performance Improvement
### What problem does this PR solve?
Add OceanBase doc engine. Close#5350
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
