fix(input): correct IME candidate popup positioning by clamping composition range

[cnaron]

Problem

When typing with an input method editor (such as Chinese Pinyin, Japanese, or other CJK IMEs) on macOS, the candidate selection window frequently pops up at the bottom-left corner of the screen (0, 0) instead of following the text insertion caret.

Root Cause

During IME composition:

1. macOS NSTextInputClient calls firstRectForCharacterRange: synchronously right after setMarkedText:selectedRange:replacementRange:.
2. At this moment, GPUI's frame render loop has not yet produced a new TextLayout for the newly inserted/replaced composition text, so self.last_layout only reflects the text before the current composition step.
3. As a result, the requested range's end index range.end can exceed layout.len(). Previously, layout.position_for_index(range.end)? failed and returned None.
4. In crates/gpui_macos/src/window.rs, first_rect_for_character_range maps None to NSRect::new(NSPoint::new(0., 0.), NSSize::new(0., 0.)) (the bottom-left of the display).

Solution

• Clamp start_idx and end_idx to layout.len() so that position_for_index safely resolves to the valid insertion point even if layout has not refreshed yet.
• If last_layout is not available, return a fallback caret rect anchored at bounds.origin instead of returning None.
• Ensure single-line bounds calculate width as (end.x - start.x).max(px(2.0)) with proper line_height.

Tested on macOS with Chinese Pinyin IME; candidate window now consistently follows the insertion caret.