Fix live preview inline caret boundary

[jsgrrchg]

Summary

Fixes a live preview caret visibility issue when the caret sits at the closing boundary of inline Markdown formatting, such as *italic text.*|.

Root Cause

Inline formatting reveal logic treated active ranges as half-open. A caret exactly at the end of an emphasis token did not activate the token, so the closing delimiter remained hidden as a zero-width inline decoration. That could make the caret appear hidden at the end of formatted text.

Changes

• Add boundary-aware range detection for selection activity.
• Use the boundary-aware strategy for inline formatting decorations.
• Keep existing half-open behavior for links, highlights, and other token types.
• Add a regression test for emphasis delimiters at the closing caret boundary.

Validation

• npm test -- --run src/features/editor/extensions/livePreviewBlocks.test.ts src/features/editor/extensions/livePreviewInline.test.ts
• git diff --check