Flaky: HttpTimeoutTest failures across multiple native platforms

## Description
Multiple `HttpTimeoutTest` methods fail intermittently across native platforms:

1. `testSocketTimeoutWriteFailOnWrite` — `UncompletedCoroutinesError` on Darwin engine (macOS X64)
2. `testGetRequestTimeoutWithSeparateReceive` — Curl/CIO request timeout (mingwX64)
3. `testSocketTimeoutRead` — timeout flake (mingwX64)

## Impact
These contribute to CI failures on Native macOS X64 and Native Windows X64 checks.

## Affected PRs (sample)
- #5426, #5407, #5381

## Platforms
- Native macOS X64
- Native Windows X64 (mingwX64)

## Suggested investigation
- The `UncompletedCoroutinesError` suggests coroutines are leaking from the test scope
- Timeout flakes may need increased timeouts or better cancellation handling in the test harness

Start with the HttpTimeoutTest methods named in the issue and reproduce them on the Native macOS X64 and mingwX64 targets. Trace the UncompletedCoroutinesError and Curl/CIO timeout failures through the test harness and cancellation handling. Done means the affected tests pass reliably on both platforms without leaked coroutines or intermittent timeout failures.

Written by the indexing model from the issue text.