editor: toggle inline formats off at closing marker

[junereycasuga]

Summary

Fixes inline formatting shortcuts so pressing Mod+B / Mod+I a second time after typing formatted text exits the formatting span instead of inserting another nested markdown marker pair.

Changes

• Detects empty-cursor formatting toggles immediately before a closing marker.
• Moves the cursor after the closing marker to “turn off” the active format.
• Removes an untouched empty marker pair when toggled again immediately.
• Adds regression coverage for active-format exit behavior and empty-pair cleanup.

Before

Cap.2026-07-22.at.08.32.02.mp4

After

Cap.2026-07-22.at.08.30.35.mp4