ClientWebSocket Loopback Server tests

[CarnaViire]

UPD: all related CI passed (innerloop+outerloop+wasm)

Fixes #73849
Fixes #28957

[WIP until CI validation on all platforms]

Expanding test coverage by bringing all of the OuterLoop external server tests to also run in inner loop against the Loopback server.

• reusing same echo handler code from the external server
• adding HTTP/2 to test matrix
• unifying hierarchical test structure to ensure full matrix coverage
• enabled some previously disabled tests
• fixed test loopback infra bugs for HTTP/2 websockets

Innerloop tests before:

Test Run Successful.
Total tests: 181
     Passed: 181
 Total time: 3.2344 Seconds

Innerloop tests after (181->661, +265%):

Test Run Successful.
Total tests: 682
     Passed: 661
    Skipped: 21
 Total time: 16.4301 Seconds

Known gaps:

• hierarchical structure downsides (e.g. having to "duplicate" subclass definitions).
• xUnit trait categorization limitations (same test cannot be both outerloop and non-outerloop on the same platform)
• 2 tests are temporarily disabled (only on HTTP/2) due to timeouts, requires further investigation
• Tried to keep reshuffling to a minimum, but some was necessary for logical grouping (e.g. external vs loopback separation)

[CarnaViire]

/azp run runtime-libraries-coreclr outerloop

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[ManickaP]

Only poking my nose a bit in stuff I know 😄

[CarnaViire]

/azp run runtime-libraries-coreclr outerloop

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[Copilot]

Pull Request Overview

This PR expands WebSocket test coverage by running external “OuterLoop” tests in the inner loop against a loopback server, adds HTTP/2 support, and unifies the test class hierarchy.

• Refactored test data sources (EchoServers, EchoServersAndBoolean) to use Uri[] and streamlined MemberData generation.
• Consolidated WebSocket setup into WebSocketHelper (added ConnectAsync, TestEcho, and UTF-8 helpers) and removed WebSocketData.
• Updated .csproj files to include new loopback test files and removed obsolete ones.

Reviewed Changes

Copilot reviewed 44 out of 44 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Net.WebSockets/tests/WebSocketCreateTest.cs	Refactored echo server MemberData to use Uri[] and boolean combinations
src/libraries/System.Net.WebSockets.Client/tests/WebSocketHelper.cs	Added ConnectAsync, TestEcho, and ToUtf8/FromUtf8; removed WebSocketData
src/libraries/System.Net.WebSockets.Client/tests/ClientWebSocketTestBase.cs	Unified MemberData generation, added HTTP/2 query-header handling

Comments suppressed due to low confidence (3)

src/libraries/System.Net.WebSockets.Client/tests/ClientWebSocketTestBase.cs:18

• [nitpick] Consider renaming EchoServers_Values to something like EchoServerUris for clearer intent and to follow C# naming conventions without underscores.

        public static readonly Uri[] EchoServers_Values = System.Net.Test.Common.Configuration.WebSockets.GetEchoServers();

src/libraries/System.Net.WebSockets.Client/tests/LoopbackServer/LoopbackWebSocketServer.cs:135

• [nitpick] The Options record has grown to many properties; consider splitting HTTP/1.1 vs HTTP/2 options into separate types or using nested builders to improve clarity and reduce complexity.

        public record class Options()

src/libraries/System.Net.WebSockets.Client/tests/WebSocketHelper.cs:189

• [nitpick] Calling Encoding.UTF8.GetBytes allocates a new byte array on each call; consider reusing a shared Encoding instance or using a pooled buffer for high-volume or large-message scenarios.

        public static ArraySegment<byte> ToUtf8(string text)

[ManickaP]

Just starting 😄
Publishing what I have so far, so it won't get lost.

[ManickaP]

More feedback. Most is opinionated so don't take it as some directive.

[ManickaP]

I'm not poking into this more.

[CarnaViire]

/ba-g (unrelated) wasm timeout is already tracked by dotnet/dnceng#3008