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]

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])))]);

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