Flaky test: HttpClientIdleTimeoutTest.testRequestIdleTimeout() on H3_QUICHE — fails in 20% of CI builds

**Jetty version(s)**
12.1.x (12.1.8-SNAPSHOT through 12.1.9-SNAPSHOT)

**Jetty Environment**
core (client transport tests)

**HTTP version**
HTTP/3 (H3_QUICHE)

**Java version/vendor**
JDK 17, 21, 25

**OS type/version**
Linux (Jenkins CI)

**Description**

`HttpClientIdleTimeoutTest.testRequestIdleTimeout()` is a frequently flaky test on the `jetty-12.1.x` CI branch when running with the `H3_QUICHE` transport. Over the last 60 builds (#1181–#1240), it failed as a flaky failure in **12 builds (20%)**.

**Typical failure:**

```
org.opentest4j.AssertionFailedError: expected: <200> but was: <500>
at HttpClientIdleTimeoutTest.testRequestIdleTimeout(HttpClientIdleTimeoutTest.java:125)
```

The related test `testClientIdleTimeout()` also fails on H3_QUICHE with the same pattern in **7 builds (12%)**, suggesting a systemic issue with idle timeout handling on the HTTP/3 QUICHE transport.

**Frequency data (from Jenkins flaky test report):**

- `testRequestIdleTimeout`: 12 out of 60 builds (20% failure rate)
- `testClientIdleTimeout`: 7 out of 60 builds (12% failure rate)
- Failures are exclusively on the `H3_QUICHE` transport variant

**Jenkins job:** https://jenkins.webtide.net/job/jetty.project/job/jetty-12.1.x/