[HTTP] Fix HttpListener tests blocking xunit workers (#21870)

[ManickaP]

Replace blocking patterns that cause deadlocks when xunit parallel execution is enabled:

• Replace .Result with await on task completions
• Replace Task.Run(() => GetContext()) with GetContextAsync()
• Replace Task.Run(() => Receive()) with ReceiveAsync()
• Convert synchronous Socket.Send() to SendAsync() in async methods
• Re-enable parallel test execution by removing DisableTestParallelization and MaxParallelThreads = 1
• Fix disposal order in HttpListenerRequestTests to close the client socket before the listener, avoiding a 1s linger timeout per test

Fixes #21870

[Copilot]

Pull request overview

This PR updates System.Net.HttpListener tests to avoid blocking xUnit worker threads (e.g., .Result, Task.Run around sync socket APIs), so the suite can run reliably when xUnit parallel execution is enabled.

Changes:

• Replace blocking wait patterns with await, and replace Task.Run(() => ...) wrappers with native async APIs (GetContextAsync, socket async APIs).
• Convert synchronous socket sends to SendAsync inside async test flows.
• Remove the assembly-level DisableTestParallelization/MaxParallelThreads = 1 restriction and adjust disposal ordering to reduce per-test timeouts.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/libraries/System.Net.HttpListener/tests/XUnitAssemblyAttributes.cs	Removes forced disabling of parallelization at the assembly level.
src/libraries/System.Net.HttpListener/tests/InvalidClientRequestTests.cs	Switches send/receive to async and avoids .Result (but currently has a Task/ValueTask mismatch).
src/libraries/System.Net.HttpListener/tests/HttpResponseStreamTests.cs	Uses SendAsync instead of blocking Send in async tests.
src/libraries/System.Net.HttpListener/tests/HttpRequestStreamTests.cs	Uses SendAsync instead of blocking Send in async tests.
src/libraries/System.Net.HttpListener/tests/HttpListenerResponseTests.cs	Uses SendAsync instead of blocking Send when establishing contexts.
src/libraries/System.Net.HttpListener/tests/HttpListenerRequestTests.cs	Fixes disposal order and replaces .Result with await; uses SendAsync in request helper.
src/libraries/System.Net.HttpListener/tests/HttpListenerContextTests.cs	Uses SendAsync instead of blocking Send when establishing contexts.
src/libraries/System.Net.HttpListener/tests/HttpListenerAuthenticationTests.cs	Removes Task.Run(() => GetContext()) + cancellation token pattern; uses GetContextAsync and await for client responses.

[wfurt]

LGTM

[ManickaP]

/ba-g #124515