refactor(connections): unify schema tree building across Postgres, MySQL and SDUI drivers #1505

Problem

In lib/features/connections/, schema tree rendering for database objects has evolved into separate massive implementations:

• connections_panel_pg_tree.dart (959 lines)
• connections_panel_mysql.dart (782 lines)
• Meanwhile, extension drivers use a declarative Server-Driven UI model (sdui_tree_builder.dart).

Both Postgres and MySQL tree files implement almost identical boilerplate:

1. Recursive node expansion / collapse tracking.
2. Async lazy-loading spinners and error retry handling.
3. Node context menu creation (query, copy name, refresh, drop).
4. Schema object icon caching and indent guide rendering.

This duplication inflates bundle size, leads to inconsistent UI behavior across engines, and increases maintenance costs when adding new database features.

Scope

• Extract a unified DatabaseTreeNode / SchemaTreeAdapter abstraction in features/connections/.
• Consolidate node lazy-loading, expansion state, error banners, and context menu rendering into a shared tree controller.
• Refactor PostgreSQL and MySQL trees to supply metadata definitions to the common tree adapter instead of implementing distinct widget trees from scratch.

Acceptance

• connections_panel_pg_tree.dart and connections_panel_mysql.dart reduced by at least 50% in line count.
• Unified node interaction and context menu behavior across all database drivers.
• Schema tree regression tests remain green.