fix: stabilize flaky ParamFlowDefaultCheckerTest by using mocked time

[EvanYao826]

Summary

The test testParamFlowDefaultCheckSingleValueCheckQpsMultipleThreads was flaky because it used real time (TimeUnit.sleep and wall-clock-based loops) for concurrent assertions. This caused race conditions when the test ran in isolation vs. after other tests.

Root Cause

The test's second phase used TimeUtil.currentTimeMillis() in a busy loop with real thread sleep, making the assertion result depend on:

• Exact thread scheduling timing
• Whether a previous test's time mock was still active
• Wall-clock drift during the 2.5-second loop

Fix

• Wrapped the entire test in MockedStatic<TimeUtil> (consistent with all other tests in this class)
• Replaced the real-time busy loop with a clean second round of concurrent requests after advancing the mock clock past the duration window
• This makes the test deterministic regardless of thread scheduling or test execution order

Fixes #2426