Restore tree filter behavior and API signature (remove slash-prefix handling)

[koianaoki]

Motivation

• Restore the tree filter behavior to match the main-branch expectation where opening the filter does not pre-populate a leading / and the API remains a no-argument action.
• Keep the filter display and matching logic consistent for empty queries so the UI shows an empty filter count state rather than preserving a slash prefix.

Description

• Reverted action_tree_filter signature back to def action_tree_filter(self) -> None and initialize self._tree_filter_text to an empty string in sqlit/domains/explorer/ui/mixins/tree_filter.py.
• Removed the special-case handling that opened the filter with a / (calls now use self.action_tree_filter() instead of passing "/") in the same mixin's key handling logic.
• When the query is empty, tree_filter_input.set_filter now receives an empty string for the filter text (restoring prior display behavior).
• Updated the explorer protocol sqlit/shared/ui/protocols/explorer.py to match the restored no-argument signature and removed the tests that asserted the slash-prefix behavior in tests/unit/test_tree_filter.py.

Testing

• Ran the unit tests for the tree filter with uv run pytest tests/unit/test_tree_filter.py and all tests passed (5 passed).

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Codex Task