text_view: Add range highlights

[white07S]

Refs #3214

Description

Adds range highlights to the Markdown TextView. This covers highlighting in #3214; scrolling to a range will follow in a separate PR.

Applications can search rendered_text() and paint results or citations behind the rendered text without reparsing the document. Ranges use UTF-8 byte offsets into the text produced by plain copy, so a search for hello world finds hello **world**, and repeated text is identified by position. Highlights do not change layout, selection, or copy behavior.

Highlights remain on unchanged text when content updates, including appended text during streaming. After an edit inside a table, cells in and after the edited row lose their highlights because cells are identified by position. The Markdown example now has a “Find in preview” field and match count to demonstrate the API.

Public API

Added in gpui-base and re-exported from gpui_component::text:

• TextViewState::rendered_text(&self) -> RenderedText gives applications the visible text to search when computing highlight ranges. The setter itself reads the current text internally.
• TextViewState::set_range_highlights(&mut self, highlights: impl IntoIterator<Item = RangeHighlight>, cx: &mut Context<Self>) -> Result<(), RangeHighlightError> replaces highlights for the current rendered text; compute the ranges and set them in the same state update.
• TextViewState::clear_range_highlights(&mut self, cx: &mut Context<Self>) removes all highlights.
• RenderedText provides as_str(&self) -> &str, len(&self) -> usize, and is_empty(&self) -> bool. Its view/revision identity is used only for equality, allowing observers such as the Markdown example to skip searching unchanged text again.
• RangeHighlight::new(range: Range<usize>, background: impl Into<Hsla>) -> Self creates a highlight with a required background color; range(&self) -> Range<usize> and background(&self) -> Hsla expose its values.
• RangeHighlightError reports Unsupported (HTML views) or InvalidRange(usize); rejected updates leave existing highlights intact.

How to Test

• Run cargo test -p gpui-base --lib text:: for rendered-text, range validation, update retention, and paint behavior.
• Run cargo run -p example-markdown, then type into “Find in preview” at the bottom. Matches should be highlighted in the preview and the count should update.

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)

[huacnlee]

@white07S I simplified the range highlight API in 1c2458b: removed set_range_highlights_for_snapshot and its stale/foreign errors, kept rendered_text() for computing offsets in the rendered text, and updated the docs and PR description. The gpui-base text tests and Markdown example check pass. Could you review the updates and let me know if the API matches what you intended?

[white07S]

@huacnlee Thanks, that matches what I intended, and dropping the snapshot argument makes it simpler to use. Two small things: the RangeHighlight doc comment now ends mid-sentence ("…paints over the earlier. A"), and it might be worth one line in the docs that an app searching outside the update can compare rendered_text() before setting. I'll update #3216 so reveal_range follows the same shape (reveal_range(range, cx)).