Improve query and UI performance

[shobhit-seg]

Summary

• reuse ClickHouse result metadata to eliminate redundant DESCRIBE TABLE requests
• cache and debounce table search, reduce render-loop work, and optimize result rendering
• close stale and redundant clients, extend default limits to CTE selects, and remove the query Format button

Measured impact

• query task: 4.956 ms to 2.505 ms in the controlled latency benchmark
• Explorer fetch: 4.978 ms to 2.500 ms
• six rapid table-search keystrokes: six rebuilds to one
• idle resizer theme bindings over 600 frames: 6000 to zero

Validation

• 119 tests passed
• Ruff lint and format checks passed
• compileall and git diff --check passed
• DearPyGUI toolbar and row-clipper smoke tests passed

[abhi204]

🤖 AI pre-review — posted by Claude Code on behalf of @abhi204 (model: claude-opus-4-8). Not a formal review; @abhi204 still signs off.

No blocking issues found. Checked for bugs, security, and CLAUDE.md compliance.

Reviewed the connection-lifecycle rework in database.py (client close paths, lock-guarded disconnect, pool race handling, close_all), the resizer theme state-change optimization and CTE-aware _add_default_limit in query_interface.py, the result-metadata reuse (get_result_column_types), and the table-list caching/debounce in table_browser_ui.py. All changes are correct and well-covered by the added tests.