Fix preserving messages for stateful reconnect with backplane

[BrennanConroy]

Fixes #55575

Changed MessageBuffer.WriteAsync methods to use the hub message type since that's the only information we're using in the method. And updated the SerializedHubMessage usage to assume we're sending a HubInvocationMessage since that's the only message type we currently use with SerializedHubMessage.

Updated tests:

• src/SignalR/server/SignalR/test/Microsoft.AspNetCore.SignalR.Tests/Internal/MessageBufferTests.cs: Added a regression test to ensure unacknowledged serialized messages are resent on reconnect.

• src/SignalR/server/StackExchangeRedis/test/RedisEndToEnd.cs: Added a new E2E stateful reconnect test to verify that messages are preserved during reconnects and introduced a WebSocketWrapper class to simulate ungraceful closures. [1] [2] [3]

[Copilot]

Pull Request Overview

This PR fixes issues with preserving messages during a stateful reconnect in a backplane scenario by updating the MessageBuffer write logic and adding comprehensive tests.

• Updated MessageBuffer.WriteAsync methods to default to HubInvocationMessage for SerializedHubMessage.
• Added a new E2E test to simulate ungraceful WebSocket closure and verify stateful reconnect.
• Introduced a StatefulHub and integrated it into the test startup configuration.

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/SignalR/server/StackExchangeRedis/test/RedisEndToEnd.cs	Added a new E2E test for stateful reconnect with a custom WebSocketWrapper.
src/SignalR/server/SignalR/test/Microsoft.AspNetCore.SignalR.Tests/Internal/MessageBufferTests.cs	Added a regression test to validate that unacknowledged serialized messages are resent on reconnect.
src/SignalR/server/StackExchangeRedis/test/StatefulHub.cs	New hub implementation to support stateful reconnect testing.
src/SignalR/server/StackExchangeRedis/test/Startup.cs	Updated endpoints mapping to support the new StatefulHub with stateful reconnect enabled.
src/SignalR/server/Core/src/SerializedHubMessage.cs	Added a Debug.Assert to ensure the message type is compatible with HubInvocationMessage.
src/SignalR/common/Shared/MessageBuffer.cs	Modified WriteAsync methods to use the message type instead of the message instance for better type clarity.

Comments suppressed due to low confidence (1)

src/SignalR/server/SignalR/test/Microsoft.AspNetCore.SignalR.Tests/Internal/MessageBufferTests.cs:183

• The use of square brackets for the InvocationMessage arguments appears to be a syntax error. Consider replacing [1] with a proper array initializer (e.g. new object[] { 1 }).

var invocationMessage = new SerializedHubMessage([new SerializedMessage(protocol.Name, protocol.GetMessageBytes(new InvocationMessage("method1", [1])))]);

[dotnet-policy-service]

Looks like this PR hasn't been active for some time and the codebase could have been changed in the meantime.
To make sure no conflicting changes have occurred, please rerun validation before merging. You can do this by leaving an /azp run comment here (requires commit rights), or by simply closing and reopening.

[halter73]

Do we want to backport this?

[BrennanConroy]

Yeah, looks like a decent number of people have hit this.