[infra] Flaky Windows CI failure in dart test --coverage ("Connection closed before full header was received") #3738

On the build (windows, dev) bot, dart test --coverage=./coverage flakily fails to load a random test suite with a VM service WebSocket error:

Failed to load "<test_file>.dart": Connection closed before full header was received, uri = http://127.0.0.1:<port>/<token>=/ws

Pointers to failures

• PR [hooks_runner] Fail eagerly when file locking is not supported on file system #3679:
  • https://github.com/dart-lang/native/actions/runs/36578763370/job/114766478704 (Attempt 2)
    • Failed to load "pkgs/record_use/test/filter_test.dart": Connection closed before full header was received, uri = http://127.0.0.1:53033/o39_S1yYgVk=/ws
  • https://github.com/dart-lang/native/actions/runs/36578763370/job/109441104282 (Attempt 1)
    • Failed to load "pkgs/hooks/test/build_input_test.dart": Connection closed before full header was received, uri = http://127.0.0.1:61020/Z-u-_y79uNA=/ws
    • Hung after 2254 tests passed, 1 failed, 2 skipped. until canceled at the 6-hour GitHub Actions timeout.
  • https://github.com/dart-lang/native/actions/runs/36425428909/job/108938101019
    • Failed to load "pkgs/code_assets/test/code_assets\link_mode_test.dart": Connection closed before full header was received, uri = http://127.0.0.1:62750/oFdTJyqnfKw=/ws
    • Hung after 2198 tests passed, 1 failed, 2 skipped. until canceled at the 6-hour GitHub Actions timeout.
• main:
  • https://github.com/dart-lang/native/actions/runs/37399354778/job/112062809663
    • Failed to load "pkgs/hooks_runner/test/utils\run_process_test.dart": Connection closed before full header was received, uri = http://127.0.0.1:51706/sSf-f_tti5Y=/ws
    • Hung after 2234 tests passed, 1 failed, 2 skipped. until canceled at the 6-hour GitHub Actions timeout.

Root cause notes

1. WebSocket connection failure during suite loading:
   In package:test_core (lib/src/runner/vm/platform.dart), Configuration.debug is true whenever --coverage is passed. In VMPlatform._loadSuite, test_core calls vmServiceConnectUri(serviceWebsocket.toString()) for every test suite it loads (and package:coverage's collect() opens another WebSocket connection when each suite completes). Across ~200+ test suites running in a single dart test --coverage=./coverage invocation on Windows, the local VM service server (127.0.0.1:<port>) occasionally drops a WebSocket handshake with HttpException: Connection closed before full header was received.
2. 6-hour hang on test_core <= 0.6.20 (fixed in test_core 0.6.21):
   In test_core 0.6.20 (used in the three runs that timed out after 6 hours), VMPlatform.load only registered isolate.kill and receivePort.close to run when channel closed after a suite finished. When vmServiceConnectUri threw during load, the already-spawned test isolate and ReceivePort were never cleaned up, preventing the dart test process from exiting after all tests finished. test_core 0.6.21 wraps _loadSuite in a try/catch that runs cleanupCallbacks on load failure, so the process now exits with code 1 instead of hanging for 6 hours, though the underlying vmServiceConnectUri flake under --coverage remains.