Avoid slow parallel connect failures in SocketAsyncEventArgs test

[MihaZupan]

Fixes #132059

Summary

SocketAsyncEventArgsTest.Connect_Parallel_Fails used IPv4 and IPv6 sockets that were bound but not listening. On macOS, connecting to such sockets can remain pending rather than fail promptly, causing the test to exceed its 30-second timeout.

Use the test suite's reserved unused port instead. This still exercises parallel IPv4 and IPv6 connection failures while avoiding OS-dependent behavior for bound, non-listening ports.

Validation

• Required clr+libs baseline build passed.
• Connect_Parallel_Fails passed 50 consecutive local runs.
• All 2,500 System.Net.Sockets functional tests passed locally.
• Repeated on osx.26.arm64.open Helix using the same macOS runtime payload:
  • Original assembly, first run: 50/50 passed, taking approximately 7.85-7.94 seconds per test.
  • Updated assembly, first run: 50/50 passed, taking approximately 45-101 milliseconds per test.
  • Original assembly, second run: completed tests took approximately 19.26 seconds each; the 15-minute work item timed out during iteration 46 after 45 passes.
  • Updated assembly, second run: 50/50 passed, taking approximately 43-87 milliseconds per test and completing the work item in 32 seconds.

Across two macOS ARM64 machines per version, the original setup varied from approximately 8 to 19 seconds per completed test, while the updated test consistently completed in under 101 milliseconds. This confirms that the original setup has substantial OS-dependent latency and that using the reserved unused port removes it.

Note

This description was drafted with GitHub Copilot.

[azure-pipelines]

Azure Pipelines:
Successfully started running 3 pipeline(s).
13 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[dotnet-policy-service]

Tagging subscribers to this area: @karelz, @dotnet/ncl
See info in area-owners.md if you want to be subscribed.

[Copilot]

🟢 Approval recommended

Pull request overview

This PR updates a flaky/slow System.Net.Sockets functional test that exercises Socket.ConnectAsync(..., ConnectAlgorithm.Parallel) failure behavior, by switching the target endpoint from a bound-but-not-listening port (which can hang on macOS) to the test suite’s reserved unused port to force prompt connection refusal.

Changes:

• Replace the per-test bound sockets/PortBlocker setup with DualModeBase.UnusedPort for a predictable “connection refused” failure.
• Simplify the Socket.ConnectAsync invocation by using SocketType.Stream / ProtocolType.Tcp constants directly.

File summaries

File	Description
src/libraries/System.Net.Sockets/tests/FunctionalTests/SocketAsyncEventArgsTest.cs	Reworks Connect_Parallel_Fails to use the reserved unused port to avoid OS-dependent connect latency/timeouts (notably on macOS).

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 1
• Review effort level: Lite