feat(input): expose editor search api for custom uis

[calmondev]

Description

I'm building a project that needs a more customized code editor experience. One of the first places where I needed that customization was search. I wanted to keep the editor's matching, highlighting, scrolling, and replace behavior, but render my own search UI.

While looking into the current API, I found that search could only really be enabled or disabled through .searchable(true). The built-in panel works well, but there was no practical public API for building a custom search UI on top of the same editor behavior. This PR adds that missing API surface without exposing the built-in SearchPanel.

Added APIs include:

• set_search_query(...)
• clear_search(...)
• search_next(...)
• search_previous(...)
• search_match_count(...)
• current_search_match_index(...)
• replace_current_search_match(...)
• replace_all_search_matches(...)
  • It now converts the full replacement range through range_to_utf16(...). replace_text_in_range_silent(...) expects UTF-16 ranges, and this keeps replace-all consistent with the individual replace path and the other editor callers. For a full-document range, the previous 0..state.text.len() value would generally still clamp to the document end, so this is a consistency/safety cleanup rather than a confirmed bug fix.

Other changes:

• Search matching state now lives on InputState, so the built-in panel and custom UIs use the same search state.
• When .searchable(false) is set, the input does not register its built-in Search action handler, so parent UIs can handle Ctrl+F/Cmd+F themselves.
• SearchPanel and SearchMatcher remain private to the input module.
• The scroll-direction helpers were moved off SearchPanel into private functions in search.rs.

This PR also adds a compact custom search example in crates/story/examples/custom_search.rs and updates the English editor docs. The Chinese docs were intentionally left unchanged so the translation can be handled by someone who can validate the wording (I don't know Chinese 😞 ).

Screenshot

The custom search example:

Break Changes

None.

How to Test

Ran the focused search tests:

cargo test -p gpui-component input::search

This includes test_builtin_search_panel_updates_editor_search, which makes sure the built-in search panel still updates search state after moving the matcher state onto InputState.

Checked the custom search example compiles:

cargo check -p gpui-component-story --example custom_search

Checked the main component crate compiles:

cargo check -p gpui-component

Checked formatting/whitespace in the diff:

git diff --check

Checklist

• I have read the CONTRIBUTING document and followed the guidelines.
• Reviewed the changes in this PR and confirmed AI generated code (If any) is accurate.
• Passed cargo run for story tests related to the changes.
• Tested macOS, Windows and Linux platforms performance (if the change is platform-specific)