E2E test flaky: TestRequestParking

[yufan-su]

The E2E test TestRequestParking added in #221 was flaky in ci

Sample failures:

• https://github.com/agent-substrate/substrate/actions/runs/30657328354/job/91244870748
• https://github.com/agent-substrate/substrate/actions/runs/30477187339/job/90661727410
• https://github.com/agent-substrate/substrate/actions/runs/30476804701/job/90660441335

--- FAIL: TestRequestParking (13.43s)
    --- FAIL: TestRequestParking/ParkThenServed (6.11s)
    --- PASS: TestRequestParking/BudgetExhaustion (5.06s)
FAIL