ci: isolate dropped-language-gap.test.ts as a regression canary (#1147) (#1169)

Add a dedicated step in the engine-parity job that runs only
`tests/integration/dropped-language-gap.test.ts`. The broader `test`
job already runs this file as part of `npm test`, but a failure there
gets buried in 500+ test names. The standalone step surfaces a clear
"Dropped-language gap regression guard" status on every PR and rules
out test pollution from neighbouring files when the canary fires.

The gap-repair path (#1083) is load-bearing for engine parity but
silently breaks if `backfillNativeDroppedFiles` ever stops writing the
file node row — exactly the shape #1147 reported.

Refs #1147

Author: carlos-alm

.github/workflows/ci.yml

@@ -284,6 +284,18 @@ jobs:
           CODEGRAPH_PARITY: '1'
         run: npx vitest run tests/engines/ tests/integration/build-parity.test.ts --reporter=verbose
 
+      # Isolated regression canary for #1147 — runs only the dropped-language-gap
+      # suite so a failure surfaces under a clear step name and can't be masked
+      # by pollution from neighbouring tests in the larger `test` job. The gap
+      # repair path (#1083) is load-bearing for engine parity but easy to break
+      # silently — the bug shape is "node row never re-inserted", which only
+      # this test exercises directly.
+      - name: Dropped-language gap regression guard (#1147)
+        shell: bash
+        env:
+          CODEGRAPH_PARITY: '1'
+        run: npx vitest run tests/integration/dropped-language-gap.test.ts --reporter=verbose
+
   rust-check:
     runs-on: ubuntu-latest
     name: Rust compile check