fix(security): polynomial ReDoS in comboAgentMiddleware regex → main (#3983)

Brings the release/v3.8.27 fix (#3982) to main so CodeQL alerts #612/#613 close
on the next scan. Code + regression test only; the [3.8.27] CHANGELOG bullet lives
on release/v3.8.27 and reaches main when v3.8.27 ships (identical file → no merge
conflict). Detection pattern drops the unbounded surrounding newline run; global
strip pattern bounds it ({0,16}). Behavior unchanged (107 related tests green).

tests/unit/combo-omnimodel-tag-stripping.test.ts

@@ -3,6 +3,7 @@ import assert from "node:assert/strict";
 import {
   stripModelTags,
   injectModelTag,
+  extractPinnedModel,
 } from "../../open-sse/services/comboAgentMiddleware.ts";
 
 // Regression for the non-global CACHE_TAG_PATTERN bug: `.replace()` with a
@@ -56,3 +57,45 @@ describe("comboAgentMiddleware — <omniModel> tag stripping (non-global regex b
     );
   });
 });
+
+// Regression for CodeQL js/polynomial-redos (#3870): the unbounded `(?:\\n|\n|\r)*`
+// prefix/suffix on the unanchored CACHE_TAG_PATTERN made `.test()` / `.replace()`
+// run in O(n²) on inputs with many newlines. The fix drops the surrounding runs from
+// the detection pattern and bounds them ({0,16}) in the global strip pattern.
+describe("comboAgentMiddleware — ReDoS safety + newline-wrapped tags (#3870)", () => {
+  test("stripModelTags stays linear on a long run of newlines (no tag present)", () => {
+    const evil = "\n".repeat(50_000);
+    const start = process.hrtime.bigint();
+    const stripped = stripModelTags([{ role: "user", content: evil }]);
+    const elapsedMs = Number(process.hrtime.bigint() - start) / 1e6;
+    assert.equal(stripped[0].content, evil, "no tag → content returned unchanged");
+    assert.ok(elapsedMs < 1000, `regex must stay linear; took ${elapsedMs.toFixed(1)}ms`);
+  });
+
+  test("stripModelTags stays linear on many newlines before a never-closing tag", () => {
+    const evil = "\n".repeat(50_000) + "<omniModel" + "x".repeat(2000);
+    const start = process.hrtime.bigint();
+    stripModelTags([{ role: "user", content: evil }]);
+    const elapsedMs = Number(process.hrtime.bigint() - start) / 1e6;
+    assert.ok(elapsedMs < 1000, `regex must stay linear; took ${elapsedMs.toFixed(1)}ms`);
+  });
+
+  test("extractPinnedModel still finds a tag wrapped in newlines", () => {
+    const messages = [
+      {
+        role: "assistant",
+        content: "answer\n\n<omniModel>claude/claude-opus-4-8</omniModel>\n\n",
+      },
+    ];
+    assert.equal(extractPinnedModel(messages), "claude/claude-opus-4-8");
+  });
+
+  test("stripModelTags removes the newline run wrapping a tag (no blank line left)", () => {
+    const out = String(
+      stripModelTags([{ role: "user", content: "before\n\n<omniModel>a/b</omniModel>\n\nafter" }])[0]
+        .content
+    );
+    assert.ok(!out.includes("<omniModel>"), "tag removed");
+    assert.ok(!out.includes("\n\n\n"), "no triple newline left from stripping");
+  });
+});