Resolves #61888.

Multi-modifier gestures could leave GPUI’s standalone-modifier state
armed after one modifier was released. For example, releasing `Alt`
before `Shift` during an `Alt+Shift` gesture caused the final `Shift`
release to be synthesized as a standalone shift keystroke.

This diff invalidates standalone-modifier synthesis whenever multiple
modifiers are active, while preserving genuine modifier-only key
bindings.

Release Notes:
- Fixed Alt+Shift gestures potentially triggering standalone Shift key
bindings.