test(service-ai): add buildContextSchemaMessages to the quota-test mock runtime

The mock AgentRuntime in agent-chat-quota.test.ts didn't implement the new
buildContextSchemaMessages method, so the route's await on it threw and the
handler returned 500 instead of 200. Mirror the real runtime in the mock.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: os-zhuang

packages/services/service-ai/src/__tests__/agent-chat-quota.test.ts

@@ -120,6 +120,7 @@ function mockAgentRuntime(): AgentRuntime {
     })),
     resolveActiveSkills: vi.fn(async () => []),
     buildSystemMessages: vi.fn(() => [{ role: 'system', content: 'sys' }]),
+    buildContextSchemaMessages: vi.fn(async () => []),
     buildRequestOptions: vi.fn(() => ({})),
     listAgents: vi.fn(async () => []),
   } as unknown as AgentRuntime;