Preserve leading '/' when opening tree filter and add initial_text parameter

[koianaoki]

Motivation

• Ensure that opening the tree filter via the / key preserves the leading slash in the input and enters regex mode so users can immediately type a regex prefix.
• Allow callers to open the tree filter with optional initial text to support explicit invocations with a starting character.

Description

• Added an initial_text: str | None = None parameter to action_tree_filter and updated the ExplorerActionsProtocol signature to match.
• When initial_text is not provided, infer a leading slash if the filter was opened by the / key using _last_key_char or _last_key, and set _tree_filter_text to initial_text instead of clearing it.
• Updated key handling to call action_tree_filter("/") when receiving / key events and adjusted printable character handling accordingly.
• Use the raw filter text when calling tree_filter_input.set_filter for empty queries so the leading slash remains visible.
• Added unit tests covering opening the filter with / and preserving an empty regex prefix.

Testing

• Added test_tree_filter_opened_by_slash_keeps_slash_visible which asserts the leading slash is preserved and the filter enters regex mode. This test passed.
• Added test_tree_filter_empty_regex_prefix_remains_visible which asserts an empty regex prefix ("/") sets regex mode and updates the displayed filter; this test passed.
• Existing tree filter unit tests were run and continued to pass (including invalid-regex handling and regex prefix tests).

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Codex Task