ui(theme): replace hardcoded material colors with QueryaWorkbenchTheme status tokens #1643

Problem

Several UI components hardcode Material color constants instead of resolving tokens from QueryaWorkbenchTheme or ColorScheme:

• lib/features/connections/ssh_tunnel_section.dart: uses material.Colors.green for successful test connection messages while using cs.destructive for failures.
• lib/features/connections/extension_connection_form.dart: uses material.Colors.green for successful connection tests while using theme.destructive for failures.
• lib/features/connections/querya_connection_tree_row.dart: uses material.Colors.amber.shade600 for the pinned star icon and context menu item instead of wb.warning.
• lib/features/workspace/grid_cell_editor.dart: uses material.Colors.red.shade500 for validation error border and tooltip icon instead of cs.destructive.
• lib/features/workspace/dml_preview_dialog.dart: uses amber.shade700, green.shade600, and red.shade600 for mutation breakdown badges instead of theme tokens.

These colors do not adapt across dark, light, or custom workbench themes and can lead to low contrast.

Change

Consistently use context.workbench.success, context.workbench.warning, and context.colorScheme.destructive across all status indicators and badges.