Closes #679

Summary of Changes

• Multi-Tab Sessions Architecture:
  • Added SqlQueryTabSession encapsulating query text (TextEditingController), top/bottom split fraction (ValueNotifier<double>), columns, rows, execution status, and in-memory DataGridStagingBuffer.
  • Added SqlQueryTabBar powered by QueryaTabStrip with dynamic close buttons on individual tabs and a + add tab button.
  • Implemented showUnsavedTabChangesDialog to guard against losing uncommitted staging changes when closing dirty query tabs.
  • Preserved tab state across switching via material.IndexedStack.
• Workspaces Converted:
  • PostgresSqlWorkspace
  • MysqlSqlWorkspace
  • SqliteSqlWorkspace
  • ExtensionSqlWorkspace
• Keyboard Shortcuts & Bridge:
  • Added intents and shortcuts for Ctrl+T / Cmd+T (new tab), Ctrl+W / Cmd+W (close tab), Ctrl+Tab (next tab), and Ctrl+Shift+Tab (previous tab), including Russian keyboard layout support.
  • Integrated with SqlEditorCommandBridge.
• Automated Verification:
  • Added unit and widget tests in test/features/workspace/multi_tab_sql_workspace_test.dart (100% passing).
  • Clean dart analyze with 0 issues.