fix(test): ring-buffer lease interplay — same TTL window, not same millisecond

Two back-to-back mintLease() calls each stamp Date.now() + TTL; when they
straddle a millisecond boundary the exact-equality assertion flakes
(observed in CI: expiries of ...525 vs ...526). Assert the expiries are
within a 50 ms window instead — the invariant under test is that leases
share a TTL policy, not that they mint in the same clock tick.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

browse/test/terminal-agent-ring-buffer-runtime.test.ts

@@ -148,7 +148,10 @@ describe('lease lifecycle interplay (via pty-session-lease)', () => {
     const vb = validateLease(b.sessionId);
     expect(va.ok && vb.ok).toBe(true);
     if (va.ok && vb.ok) {
-      expect(va.expiresAt).toBe(vb.expiresAt);
+      // Same TTL window, not same millisecond: each mint stamps
+      // Date.now() + TTL, and back-to-back calls can straddle a ms boundary
+      // (observed in CI: ...525 vs ...526). Exact equality is a timing flake.
+      expect(Math.abs(va.expiresAt - vb.expiresAt)).toBeLessThanOrEqual(50);
     }
   });
 });