test(chatcore): dedicated unit tests for 6 leaves + wire into stryker mutate (QG v2 Fase 9 T5 Fase 3) (#4218)

Integrated into release/v3.8.29 (QG v2 Fase 9 T5 Fase 3 — dedicated leaf unit tests + stryker batch h).

.github/workflows/nightly-mutation.yml

@@ -23,9 +23,11 @@ jobs:
     #     combo/* leaves are back in `mutate` (covered by the 24 combo-*.test.ts), grouped into
     #     2 batches (d=heavy, e=light). After #4204 (D7b) merged, the reset-aware quota pair
     #     quotaScoring/quotaStrategies was added as batch f. A covering-test audit then added the
-    #     6 chatCore/* leaves with direct unit coverage as batch g; the other 9 chatCore leaves
-    #     remain a follow-up — see _mutate_godfiles_excluded_comment in stryker.conf.json.
-    # 7 PARALLEL batches, each overriding the mutate set via `--mutate` (Stryker 9 CLI:
+    #     6 chatCore/* leaves with direct unit coverage as batch g. A follow-up then wrote dedicated
+    #     unit tests for 6 more leaves and added them as batch h; the last 3 chatCore leaves
+    #     (memorySkillsInjection/semanticCache/telemetryHelpers) remain a follow-up — see
+    #     _mutate_godfiles_excluded_comment in stryker.conf.json.
+    # 8 PARALLEL batches, each overriding the mutate set via `--mutate` (Stryker 9 CLI:
     # `-m, --mutate <comma-list>`; the conf's `mutate[]` remains the local-run default/union).
     # Each batch targets <180min; full coverage every night in parallel (~180min wall-clock).
     # The `incremental` cache (per-batch key) makes runs after the first cold one much cheaper,
@@ -50,6 +52,8 @@ jobs:
             mutate: "open-sse/services/combo/quotaScoring.ts,open-sse/services/combo/quotaStrategies.ts"
           - name: g
             mutate: "open-sse/handlers/chatCore/comboContextCache.ts,open-sse/handlers/chatCore/idempotency.ts,open-sse/handlers/chatCore/passthroughHelpers.ts,open-sse/handlers/chatCore/responseHeaders.ts,open-sse/handlers/chatCore/sanitization.ts,open-sse/handlers/chatCore/upstreamTimeouts.ts"
+          - name: h
+            mutate: "open-sse/handlers/chatCore/headers.ts,open-sse/handlers/chatCore/logTruncation.ts,open-sse/handlers/chatCore/memoryExtraction.ts,open-sse/handlers/chatCore/nonStreamingSse.ts,open-sse/handlers/chatCore/passthroughToolNames.ts,open-sse/handlers/chatCore/executorHelpers.ts"
     timeout-minutes: 180
     steps:
       - uses: actions/checkout@v6