fix: prevent infinite alert loop when table tab query fails

[datlechin]

Summary

• When a table tab query fails (e.g., opening pg_stat_statements_info in the extensions schema), the "Query Execution Failed" alert shows in an infinite loop — clicking OK immediately re-triggers the alert
• Root cause: handleQueryExecutionError never set lastExecutedAt on the tab. Dismissing the NSAlert sheet posts NSWindow.didBecomeKeyNotification, which re-evaluates the lazy-load predicate. Since lastExecutedAt == nil and resultRows.isEmpty, the predicate passes and runQuery() fires again
• Set lastExecutedAt = Date() in the error handler so the tab is marked as "already attempted"
• Added errorMessage == nil guard to both lazy-load predicates (didBecomeKeyNotification handler and handleTabChange) as defense-in-depth

Test plan

• Open a table that will fail (e.g., pg_stat_statements_info in extensions schema on a PostgreSQL server without shared_preload_libraries configured)
• Verify the error alert shows once and does not reappear after clicking OK
• Switch away from and back to the failed tab — verify no re-trigger
• Open a normal table tab — verify it still auto-executes correctly
• Restore tabs from persistence with a previously-failed tab — verify no loop