Flaky test: ContentTest.testEmptyContent times out on mingwX64

## Root cause

The test times out after 20 seconds on Windows native (mingwX64) with no error message. This suggests the test server or client hangs during the empty content request/response cycle on this platform.

Also observed as `ContentTest.testEmptyContent[macosArm64]` on the `release/3.x` base branch (build 412674).

## Suggested fix

Investigate whether the empty content handling has a platform-specific issue on native targets, particularly around connection handling or response completion signaling.

File: `ktor-client/ktor-client-tests/common/test/io/ktor/client/tests/ContentTest.kt`

Observed in PR #5465 and on `release/3.x` base branch.

Start with ktor-client/ktor-client-tests/common/test/io/ktor/client/tests/ContentTest.kt and run ContentTest.testEmptyContent on mingwX64, also checking the macosArm64 report. Trace the empty-content request/response cycle and connection completion; done means the test completes reliably without timing out on the affected native targets.

Written by the indexing model from the issue text.