closeBrackets auto-inserts `}}` when the user types `{{`, so the buffer
already reads `{{<prefix>}}` before a completion is accepted. The apply
callback was unconditionally appending another `}}`, producing
`{{path}}}}` (valid Mustache but obviously wrong).
Fix: peek at the two characters immediately after the completion range
and, when they're `}}`, extend the replacement range by two so the
existing closing braces are overwritten rather than left in place.
Added a regression test that drives `apply` through a real EditorView
for both the bare-prefix (no trailing `}}`) and auto-closed
(`{{prefix}}`) scenarios.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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