chore: import upstream snapshot with attribution

tests/unit/chatcore-compression-combo-predicates.test.ts

@@ -0,0 +1,64 @@
+// Characterization of the pure compression-combo predicates extracted from handleChatCore's
+// compression setup (chatCore god-file decomposition, #3501). No DB, no handler state.
+import { test } from "node:test";
+import assert from "node:assert/strict";
+
+const { isBuiltinStackedPipeline, isStackedCompressionCombo } = await import(
+  "../../open-sse/handlers/chatCore/compressionComboPredicates.ts"
+);
+
+test("isBuiltinStackedPipeline true only for the rtk(standard)→caveman(full) shape", () => {
+  assert.equal(
+    isBuiltinStackedPipeline([{ engine: "rtk" }, { engine: "caveman" }] as never),
+    true
+  );
+  assert.equal(
+    isBuiltinStackedPipeline([
+      { engine: "rtk", intensity: "standard" },
+      { engine: "caveman", intensity: "full" },
+    ] as never),
+    true
+  );
+});
+
+test("isBuiltinStackedPipeline false for wrong length / engines / intensities / config", () => {
+  assert.equal(isBuiltinStackedPipeline(undefined), false);
+  assert.equal(isBuiltinStackedPipeline([] as never), false);
+  assert.equal(isBuiltinStackedPipeline([{ engine: "rtk" }] as never), false);
+  assert.equal(
+    isBuiltinStackedPipeline([{ engine: "caveman" }, { engine: "rtk" }] as never),
+    false
+  );
+  assert.equal(
+    isBuiltinStackedPipeline([
+      { engine: "rtk", intensity: "aggressive" },
+      { engine: "caveman", intensity: "full" },
+    ] as never),
+    false
+  );
+  assert.equal(
+    isBuiltinStackedPipeline([
+      { engine: "rtk", config: { x: 1 } },
+      { engine: "caveman" },
+    ] as never),
+    false
+  );
+});
+
+test("isStackedCompressionCombo true when the combo has >= 1 pipeline layer", () => {
+  assert.equal(isStackedCompressionCombo(null), false);
+  assert.equal(
+    isStackedCompressionCombo({ id: "c", pipeline: [], languagePacks: [], outputMode: false, outputModeIntensity: "full" } as never),
+    false
+  );
+  assert.equal(
+    isStackedCompressionCombo({
+      id: "c",
+      pipeline: [{ engine: "rtk" }],
+      languagePacks: [],
+      outputMode: false,
+      outputModeIntensity: "full",
+    } as never),
+    true
+  );
+});