fix: always reset isExecuting on query completion to prevent stuck tabs

[datlechin]

Summary

• Fixes tabs permanently stuck in "executing" state after rapid refresh or query cancellation, requiring a reconnect to recover
• Root cause: isExecuting = false was guarded by capturedGeneration == queryGeneration — when a newer query superseded the current one, the stale query's cleanup was skipped entirely, leaving isExecuting = true forever
• All subsequent runQuery() calls silently returned due to the guard !isExecuting check

Changes

Ensures isExecuting, currentQueryTask, and toolbarState are always reset on every query exit path:

Path	File	Fix
Single-statement success, stale generation	MainContentCoordinator.swift	Reset isExecuting before returning
Single-statement error, stale generation	MainContentCoordinator.swift	Reset isExecuting before generation guard
Single-statement cancellation	MainContentCoordinator.swift	Added missing currentQueryTask = nil
Multi-statement inner-loop cancellation	+MultiStatement.swift	Changed break to return with rollback + cleanup (was falling through to commitTransaction)
Multi-statement inner-loop stale generation	+MultiStatement.swift	Added cleanup before return
Multi-statement error, stale generation	+MultiStatement.swift	Reset isExecuting before generation guard
Multi-statement success, stale generation	+MultiStatement.swift	Reset isExecuting before generation guard

Test plan

• Open a table tab, click Refresh (Cmd+R) rapidly multiple times — should never get stuck
• Open a table, start a slow query, immediately Refresh again — old query should be cancelled, new one executes
• Disconnect network mid-query, then Refresh — error alert should show, tab should not be stuck
• Run multi-statement SQL, cancel mid-execution — transaction should rollback, tab should not be stuck