fix(antigravity): derive stable session ID from credentials for prompt-cache reuse (#13305) (#13424)

`getAntigravitySessionId` derives a stable session id from the account key (FNV-1a over the existing `getAntigravityAccountKey`) when credentials are present, so repeated requests from one account can reuse Antigravity prompt caching (#13305). An explicit fallback id still wins, and calls without credentials still get a random id. This deliberately reverses the per-request random id from #10443; the maintainer approved the trade-off.

Maintainer addition: `credentials` is now used, so the frozen `no-unused-vars` suppression for `open-sse/services/antigravityIdentity.ts` became stale. ESLint fails on stale suppressions, so it was pruned. Antigravity suites: 34 cases green.

Validated in one consolidated batch of this series (37 PRs boarded together on `release/v3.8.51`): `typecheck:core`, `check:open-sse-typecheck` and `check:dashboard-typecheck` clean; ESLint clean on every changed file; file-size, complexity, cognitive-complexity, changelog-integrity, docs-counts, docs-sync and migration-numbering gates green (only the pre-existing `open-sse/utils/stream.ts` file-size red remains, inherited from the base); 3,743 focused `node:test` cases plus 34 vitest cases green.

Thanks @KooshaPari!

config/quality/eslint-suppressions.json

@@ -396,11 +396,6 @@
       "count": 1
     }
   },
-  "open-sse/services/antigravityIdentity.ts": {
-    "@typescript-eslint/no-unused-vars": {
-      "count": 1
-    }
-  },
   "open-sse/services/antigravityProjectBootstrap.ts": {
     "@typescript-eslint/no-unused-vars": {
       "count": 1

open-sse/services/antigravityIdentity.ts

@@ -76,6 +76,7 @@ export function getAntigravitySessionId(
 ): string {
   return (
     toNonEmptyString(fallback) ||
+    deriveAntigravitySessionId(getAntigravityAccountKey(credentials)) ||
     generateAntigravitySessionId()
   );
 }

tests/unit/antigravity-dynamic-session-id-10443.test.ts

@@ -2,17 +2,39 @@ import assert from "node:assert/strict";
 import { test } from "node:test";
 import { getAntigravitySessionId } from "../../open-sse/services/antigravityIdentity.ts";
 
-test("getAntigravitySessionId yields dynamic random session IDs per request to avoid session pinning", () => {
+test("getAntigravitySessionId derives a stable session ID from credentials for prompt-cache reuse", () => {
   const credentials = { email: "user@example.com", connectionId: "conn_123" };
 
   const id1 = getAntigravitySessionId(credentials);
   const id2 = getAntigravitySessionId(credentials);
 
-  assert.notEqual(id1, id2, "getAntigravitySessionId should not pin to a static account email hash");
+  assert.equal(id1, id2, "same credentials must produce the same session ID for cache reuse");
   assert.equal(typeof id1, "string");
-  assert.equal(typeof id2, "string");
+  assert.ok(id1.length > 0, "session ID must be non-empty");
 
   const explicitFallback = "custom-session-456";
   const idWithFallback = getAntigravitySessionId(credentials, explicitFallback);
   assert.equal(idWithFallback, explicitFallback, "explicit fallback session ID should take precedence");
 });
+
+test("getAntigravitySessionId falls back to random ID when no credentials are available", () => {
+  const id1 = getAntigravitySessionId();
+  const id2 = getAntigravitySessionId();
+
+  assert.equal(typeof id1, "string");
+  assert.ok(id1.length > 0, "random session ID must be non-empty");
+
+  // Without credentials, each call should produce a different random ID
+  // (this is acceptable since there's no account to pin cache to)
+  assert.notEqual(id1, id2, "without credentials, IDs should be random per call");
+});
+
+test("getAntigravitySessionId produces different IDs for different accounts", () => {
+  const credsA = { email: "alice@example.com" };
+  const credsB = { email: "bob@example.com" };
+
+  const idA = getAntigravitySessionId(credsA);
+  const idB = getAntigravitySessionId(credsB);
+
+  assert.notEqual(idA, idB, "different accounts must produce different session IDs");
+});