input: trigger pending re-highlight when switching editor language

[Muhammad-Owais-Warsi]

Closes #2970

Description

set_highlighter() clears the cached highlighter when switching languages, but does not set the internal _pending_update flag. This means the next render never calls update_highlighter(), so the editor stays without syntax highlighting until the user edits the text (which triggers update_highlighter(force: true)).

The fix replaces cx.notify() with self.refresh(cx) — a method that already exists for exactly this purpose and sets _pending_update = true before notifying.

Screenshot

N/A — behavioral fix, no visual change unless highlighting was broken.

Break Changes

None.

How to Test

1. Run cargo run -p example-editor in the gpui-kit repo
2. Open a .rs file (Rust highlighting appears)
3. Open a .json file — highlighting should switch to JSON immediately
4. Open a .html file — highlighting should switch to HTML immediately
5. Before this fix, steps 3–4 would show stale/no highlighting until the user typed in the editor

Why the example-editor appeared to work without this fix

The example-editor calls set_value() after set_highlighter(). set_value internally sets _pending_update = true (via reset_lsp_state()), which accidentally triggers the re-highlight on the next render. This is a workaround, not correct behavior — set_highlighter() should work on its own. Every other caller that uses set_highlighter() without a subsequent set_value() hits this bug.

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)