text: Fix correct TextView selection when dragging back to the anchor line

[suxiaoshao]

Closes #2111

Description

Fix TextView selection when dragging back to the original line after crossing another line. The selection helper now treats points within the same rendered line as a single-line selection, so the highlight stays bounded by the anchor and current cursor positions. This also adds regression tests for the same visual line with different y positions.

AI-generated portions reviewed for accuracy: the selection helper adjustment and the regression tests in crates/ui/src/text/inline.rs.

Screenshot

Before

2026-03-03.19.10.59.mov

After

2026-03-03.19.07.05.mov

How to Test

1. Run cargo test -p gpui-component test_point_in_text_selection --lib.
2. In a selectable TextView, start a selection on one line, drag to the next line, then move back to the original line.
3. Verify the selection on the original line is bounded by the anchor position and the current cursor position instead of extending to the line end.

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]

Thank you.