feat(oauth): harden model family classification against gemma prefix collision

- Tighten classifyModelFamilyForQuota to require explicit gemini keyword, preventing gemma from matching Gem family.
- Add regression test in tests/oauth/oauth-account-quota-rank.test.ts.

Author: chilung-cgu

src/oauth/account-quota-rank.ts

@@ -50,9 +50,9 @@ const PASSIVE_HEADROOM_MAX_AGE_MS = 60 * 60_000;
  * Returns "Gem" for Gemini models, "Cla" for Claude/Opus/Sonnet, or undefined
  * for unknown models (which falls back to all-window ranking).
  */
-function classifyModelFamilyForQuota(modelId: string): string | undefined {
+function classifyModelFamilyForQuota(modelId: string): "Gem" | "Cla" | undefined {
   const lower = modelId.toLowerCase();
-  if (lower.includes("gemini") || lower.startsWith("gem")) return "Gem";
+  if (lower.includes("gemini") || lower === "gemini" || lower.startsWith("gemini-")) return "Gem";
   if (lower.includes("claude") || lower.includes("opus") || lower.includes("sonnet") || lower.includes("gpt-oss") || lower.includes("gpt_oss")) return "Cla";
   return undefined;
 }

tests/oauth/oauth-account-quota-rank.test.ts

@@ -354,5 +354,30 @@ describe("model-aware failover and pre-dispatch integration", () => {
     const ranked = rankAccountsByHeadroom("google-antigravity", ring, "gemini-3.8-flash");
     expect(ranked).toEqual(ring);
   });
+
+  test("does not misclassify gemma as Gemini (no Gem prefix pollution)", () => {
+    const idGem = "antigravity-gem";
+    const idCla = "antigravity-cla";
+    const ring = [idGem, idCla];
+
+    setCachedProviderAccountQuotaForTests("google-antigravity", idGem, {
+      customWindows: [
+        { label: "Gem", percent: 10 },
+        { label: "Cla", percent: 90 },
+      ],
+      updatedAt: Date.now(),
+    });
+    setCachedProviderAccountQuotaForTests("google-antigravity", idCla, {
+      customWindows: [
+        { label: "Gem", percent: 90 },
+        { label: "Cla", percent: 10 },
+      ],
+      updatedAt: Date.now(),
+    });
+
+    // When requestedModelId is gemma, it should return undefined family and compare all windows (Math.max of 10 and 90 = 90 for both)
+    const ranked = rankAccountsByHeadroom("google-antigravity", ring, "gemma-2-9b-it");
+    expect(ranked).toEqual(ring);
+  });
 });