Add integration testing guide for Wolverine.HTTP with Alba

[jeremydmiller]

Summary

Closes #2408

Adds a new guide document at /guide/http/integration-testing covering integration testing of Wolverine.HTTP endpoints using Alba and Wolverine's tracked sessions.

• Test harness setup: AppFixture + IntegrationContext base class pattern with ICollectionFixture
• TrackedHttpCall helper: How to combine Alba's Host.Scenario() with Wolverine's ExecuteAndWaitAsync() to track all message activity triggered by an HTTP request
• Real examples from the codebase: Publishing messages, cascading messages via tuple/OutgoingMessages, resource creation with Location headers, EF Core persistence verification
• ITrackedSession API reference: All RecordCollection properties (Sent, Received, Executed, etc.) and query methods (SingleMessage<T>(), MessagesOf<T>(), FindSingleTrackedMessageOfType<T>())
• TrackActivity() fluent configuration: Timeout(), DoNotAssertOnExceptionsDetected(), IncludeExternalTransports(), IgnoreMessageType<T>(), AlsoTrack(), WaitForMessageToBeReceivedAt<T>()
• Marten-specific helpers: ResetAllMartenDataFirst(), PauseThenCatchUpOnMartenDaemonActivity(), WaitForNonStaleDaemonDataAfterExecution()
• Testing mode helpers: RunWolverineInSoloMode() and DisableAllExternalWolverineTransports()
• Links to Alba docs and the existing Wolverine tracked sessions guide
• Added sidebar entry under HTTP guide section

Test plan

• Verify the new page renders in VitePress
• Verify sidebar navigation includes the new entry
• Review code samples for accuracy against current API
• Check all internal and external links resolve

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