Summary

Fixes #48725

When using the keymap editor to delete or re-bind default keybindings, two issues caused stale entries to persist:

• Deleting a default binding created a NoAction suppression entry in keymap.json (correct for GPUI-level suppression), but the keymap editor displayed it as a phantom <null> row alongside the greyed-out default. Searching by keystroke showed both entries, and clash detection counted the phantom as a real binding.

• Changing a default binding's keystroke (e.g. ⌥⌘T → ⇧⌃⌘T) added the new binding but never suppressed the original default under the old keystroke. The old default continued appearing in keystroke searches and clash warnings.

Fix

1. Filter NoAction suppression bindings from the keymap editor display. These remain in the internal binding list for conflict detection (so overridden defaults are still correctly greyed out), but are excluded from the visible match results.

2. Append a null suppression for the old keystroke when replacing a non-user binding with a different keystroke. When update_keybinding converts a Replace to Add for a non-user binding and the keystroke changes, it now also writes {"old-keystroke": null} (with the original context) to suppress the stale default.

Reproduction steps (verified fixed)

1. Open the keymap editor
2. Click Search by Keystroke and press ⌥⌘T
3. You should see "agent: new thread" and "pane: close other items" (both Default)
4. Right-click "agent: new thread" and choose Delete
5. Double-click "pane: close other items", change keystroke to ⇧⌃⌘T, click Save
6. Verify no <null> phantom row appears, and no stale defaults remain under ⌥⌘T
7. Close and reopen the keymap editor, search ⌥⌘T — no results
8. Search for "editor: wrap selections in tag" and assign ⌥⌘T — no clash warnings

Test plan

• Existing keymap_update and test_keymap_remove tests pass
• Added test: replacing a non-user binding without changing the keystroke produces no suppression
• Added test: replacing a non-user binding with context and keystroke change produces a suppression that preserves the context
• Manual verification of all reproduction steps above

Release Notes:

• Fixed keymap editor showing phantom entries and stale defaults when deleting or overriding default keybindings (#48725)