test: a mutant guard may be pytest, not only node (#329)

The registry contract demanded that every guard name a `.mjs` file, which was
true until this task added the first backend mutant —
junction-limit-backend-raw-baseline is guarded by pytest, and the mutation-gate
job already installs it. My mistake: I ran `--check` and the single mutant
after adding it, but not the unit suite that owns the registry contract, so CI
caught what I should have.

The contract keeps its point: a guard must name a file that exists.

Issue: #329
User-Visible: no

test/mutation-gate.test.mjs

@@ -32,7 +32,11 @@ test('every mutant patch anchors exactly once in the current source', () => {
 
 test('every guard command points at a file that exists', () => {
   for (const mutant of MUTANTS) {
-    const script = mutant.guard.split(' ').find((part) => part.endsWith('.mjs'));
+    // Гвард — не обязательно node: бэкенд-зеркала (#329) красит pytest, и
+    // окружение мутант-джоба его ставит. Проверяем одно: команда называет
+    // существующий файл, а не выдуманный путь.
+    const script = mutant.guard.split(' ')
+      .find((part) => part.endsWith('.mjs') || part.endsWith('.py'));
     assert.ok(script, `${mutant.id}: guard не называет исполняемый файл`);
     assert.ok(existsSync(join(repoRoot, script)),
       `${mutant.id}: guard-файла ${script} не существует`);