refactor(workspace): extract SqlWorkspaceToolbar and introduce BaseSqlExecutionDelegate #1363

Summary

1. lib/features/workspace/generic_sql_workspace.dart is 1,546 lines long and combines:
   • A complex 350+ line toolbar with 10+ controls (Execute, Explain, Cancel, session options, autocommit, timeouts).
   • Session lifecycle and tab management.
   • DML preview dialogs, destructive query guards, and staged save flows.
2. Across postgres_sql_workspace.dart, mysql_sql_workspace.dart, and sqlite_sql_workspace.dart, the SQL execution delegates repeat boilerplate logic:
   • injectSqlLimit and truncated row checks.
   • Autocommit check with automatic BEGIN transaction initiation.
   • Status message formatting (Showing first $cap row(s)... , Command completed., $n row(s).).

This redundancy complicates maintenance and conflicts with single-responsibility design (related to Epic #1151).

Proposed Solution

1. Extract the toolbar into lib/features/workspace/sql_workspace_toolbar.dart.
2. Introduce an abstract BaseSqlExecutionDelegate in lib/features/workspace/sql_execution_delegate.dart that centralizes limit injection, transaction handling, and status message resolution.
3. Have PostgreSQL, MySQL, and SQLite delegates extend BaseSqlExecutionDelegate and implement only driver-specific socket execution.

Acceptance Criteria

• GenericSqlWorkspace delegates toolbar rendering to SqlWorkspaceToolbar.
• Common execution boilerplate (limit injection, transaction state check, status line) is shared via BaseSqlExecutionDelegate.
• All database workspace unit and widget tests pass.