Explorer: add table metadata shortcuts, regex tree-filter, and MySQL index fix

[koianaoki]

Motivation

• Provide quick access to table metadata (columns and indexes) from the explorer tree to improve discovery and debugging.
• Allow more powerful tree searching by enabling regex-based tree-filter queries alongside existing fuzzy/plain modes.
• Correct MySQL index listing to include the PRIMARY index and clean up provider typing and schema-load token handling.

Description

• Added two new actions and key bindings: show_table_columns and show_table_indexes in the default keymap and updated tests to assert their presence.
• Extended TreeOnTableState footer bindings to expose the new metadata shortcuts and added TreeMixin implementations for action_show_table_columns and action_show_table_indexes with MySQL-specific SHOW FULL COLUMNS / SHOW INDEX helpers and a generic schema-service fallback; added helpers _selected_table_node_data, _is_mysql_like_provider, _format_mysql_table_reference, _show_mysql_full_columns, _show_mysql_indexes, and _index_table_matches.
• Enhanced tree filtering by adding regex mode to TreeFilterMixin with parsing of prefixes (/, re:, r:), compiled pattern caching, matching/highlighting logic, and new state fields; imported re and added protocol fields in shared/ui/protocols/explorer.py.
• Fixed MySQL index query in MySQLBaseAdapter.get_indexes to stop excluding PRIMARY and adjusted a type annotation on apply_database_override; replaced uses of setattr(self, "_schema_load_request_token", ...) with direct assignment.
• Added and updated tests to cover the new features: UI tests for metadata shortcuts and keymap, unit tests for MySQL index behavior, and unit tests for regex tree-filter behavior.

Testing

• Ran unit tests for tree filter and MySQL changes via tests/unit/test_tree_filter.py and tests/unit/test_mysql_indexes.py, and they passed.
• Ran UI-level tests for explorer metadata shortcuts and keymap provider via tests/ui/explorer/test_table_metadata_shortcuts.py and tests/ui/keybindings/test_keymap_provider.py, and they passed.
• All added and modified automated tests completed successfully in the test run.

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Codex Task