test(sse): register #8396 and #8376 unit tests in stryker tap.testFiles (#8538)

* test(sse): register #8396 and #8376 unit tests in stryker tap.testFiles

check:mutation-test-coverage fails on release/v3.8.49 at its own HEAD:
two unit tests cover mutated modules but are absent from tap.testFiles,
so their mutant kills do not count and the drift gate blocks every
PR->release run.

- tests/unit/8396-cooldown-429-cap.test.ts covers
  open-sse/services/accountFallback.ts (imports checkFallbackError)
- tests/unit/8376-econnrefused-breaker.test.ts covers
  open-sse/services/combo/comboPredicates.ts (imports
  shouldRecordProviderBreakerFailure)

Both inserted in the array's existing sorted position; no other key
touched.

* test(sse): register repro-7503-no-choices in stryker tap.testFiles

stryker.conf.json

@@ -49,6 +49,8 @@
       "tests/unit/8247-accountfallback-model-unhealthy.test.ts",
       "tests/unit/8248-accountfallback-nvidia-degraded.test.ts",
       "tests/unit/8332-combo-vision-fallback.test.ts",
+      "tests/unit/8376-econnrefused-breaker.test.ts",
+      "tests/unit/8396-cooldown-429-cap.test.ts",
       "tests/unit/account-fallback-anthropic-quota.test.ts",
       "tests/unit/account-fallback-lockout-eviction.test.ts",
       "tests/unit/account-fallback-retry-after-json.test.ts",
@@ -251,6 +253,7 @@
       "tests/unit/rate-limit-enhanced.test.ts",
       "tests/unit/rate-limit-manager.test.ts",
       "tests/unit/rate-limit-queue-timeout-lockout.test.ts",
+      "tests/unit/repro-7503-no-choices.test.ts",
       "tests/unit/repro-antigravity-404-family-cooldown-hijack.test.ts",
       "tests/unit/responses-handler.test.ts",
       "tests/unit/rotation-config-omniroute.test.ts",