Add scoped Explorer table filtering

[koianaoki]

Motivation

• When filtering while viewing a Tables folder or a table inside it, the filter should operate only on that Tables subtree instead of the whole Explorer tree.

Description

• Record a filter scope (_tree_filter_scope_path) when the tree filter is opened and introduce _get_tree_filter_scope_path / _get_tree_filter_search_root helpers to resolve the scoped subtree.
• Restrict counting, async preloading, matching, and visibility pruning to the active search root by updating _update_tree_filter, _ensure_tree_filter_search_nodes_loaded, _find_matching_nodes, _apply_filter_to_tree, and _count_all_nodes.
• Add small type/cast adjustments and extend the explorer protocol (sqlit/shared/ui/protocols/explorer.py) to expose the new scope and helper signatures required by the mixin.
• Add unit tests (tests/ui/test_tree_filter_tables_scope.py) that verify starting the filter on a Tables folder or on a table uses the Tables subtree as the filter scope.

Testing

• Ran the new UI tests with uv run pytest tests/ui/test_tree_filter_tables_scope.py -q, which passed (2 passed).
• Ran static checks with uv run ruff check ... which reported no issues.
• Ran type checks with uv run mypy ... which completed with no issues.

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Codex Task