Flaky: JettyEngineHttp2SustainabilityTest.testBigFile timeout

## Root cause

The test `io.ktor.tests.server.jetty.http2.jakarta.JettyEngineHttp2SustainabilityTest.testBigFile()` intermittently times out on JVM after 1 minute.

**Error:** `kotlinx.coroutines.test.UncompletedCoroutinesError: After waiting for 1m, the test body did not run to completion`

This appears to be a sustainability/load test that occasionally cannot complete within the coroutine test timeout, likely due to CI resource contention or Jetty HTTP/2 flow control under load.

## Reproduction

Seen in TeamCity Build All Core:
- https://ktor.teamcity.com/buildConfiguration/Ktor_KtorCore_All/412044 (PR branch)

## Impact

Intermittent failure on CI that can block unrelated PRs.

Start with io.ktor.tests.server.jetty.http2.jakarta.JettyEngineHttp2SustainabilityTest.testBigFile() and reproduce the timeout in the TeamCity Build All Core configuration or locally. Investigate the one-minute coroutine timeout and the Jetty HTTP/2 sustainability test under load; done means the test completes reliably without intermittently blocking CI.

Written by the indexing model from the issue text.