chore(bus): avoid hidden queue affinity contracts

[yordis]

• Message routing should expose its synchronization contract where every message already carries dispatch metadata.
• Focused coverage keeps serialized worker routing stable while reducing queue-routing special cases.

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📥 Commits

Reviewing files that changed from the base of the PR and between e93d90b and cae99bb.

📒 Files selected for processing (6)

• src/EventStore.Core.XUnit.Tests/Bus/MultiQueuedHandlerTests.cs
• src/EventStore.Core/Bus/MultiQueuedHandler.cs
• src/EventStore.Core/Messages/IQueueAffineMessage.cs
• src/EventStore.Core/Messages/StorageMessage.cs
• src/EventStore.Core/Messages/TcpMessage.cs
• src/EventStore.Core/Messaging/Message.cs

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🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch yordis/chore-message-synchronization-groups

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PR Summary

Medium Risk
Changes core message-to-queue routing semantics by replacing the dedicated queue-affinity interface with a new Message.SynchronizationGroup contract, which could affect ordering/affinity if any messages relied on the old hashing behavior.

Overview
MultiQueuedHandler now selects a target queue using message.SynchronizationGroup?.GetHashCode() (falling back to round-robin), removing the hidden dependency on IQueueAffineMessage.

This introduces Message.SynchronizationGroup and migrates existing affinity uses (e.g. TcpMessage.TcpSend, StorageMessage.BatchLogExpiredMessages) to override/set it, deletes IQueueAffineMessage, and adds focused xUnit coverage to keep same-group messages co-located and non-group messages round-robin.

^{Reviewed by Cursor Bugbot for commit cae99bb. Bugbot is set up for automated code reviews on this repo. Configure here.}