Make failingendpoint address non-nullable to match usage

[rbev]

Problem

FailingEndpointAddress (EF entity) and AddressOfFailingEndpoint were marked string?, but the ingestion path (FailedMessageFactory.ParseFailureDetails)
already guarantees a non-null value — it throws if the NServiceBus.FailedQ header is missing. The nullable annotation was propagated from the mutable FailureDetails DTO rather than
reflecting actual usage, leading to a nullable column, a dedicated NullableEndpointAddress migration, and defensive != null guards scattered across the EF query layer.

Changes

Made the types non-nullable with required:

• FailureDetails.AddressOfFailingEndpoint → required string (was string?)
• FailedMessageEntity.FailingEndpointAddress → required string (was string?)
• RecordedFailedProcessingAttempt.FailingEndpointAddress → required string (was string?)
• StagingMessage.FailingEndpointAddress → string record parameter (was string?)

Refactored FailedMessageFactory.ParseFailureDetails to check the FailedQ header before constructing, using an object initializer to satisfy required.

Updated FailedMessage.ProcessingAttempt.FailureDetails default from = new() to = new() { AddressOfFailingEndpoint = string.Empty } so placeholder attempts satisfy the required
member.

Removed defensive null checks that existed only because of the nullable:

• FailedMessageQueryFilters.FilterByQueueAddress — dropped FailingEndpointAddress != null && guard
• FailedMessageRetryDataStore.GetRetryPendingMessages — replaced two-branch null match with simple comparison

Added EF migration MakeFailingEndpointAddressNonNull (SQL Server + PostgreSQL) to make the column NOT NULL with defaultValue: "" to safely handle any stray nulls. Updated model
snapshots and designer files.

Fixed 5 test files that constructed FailureDetails without setting AddressOfFailingEndpoint.

What's not affected

• RavenDB — stores the domain document directly (no flattened column), no schema migration needed. Indexes and transformers read the now-non-nullable property without changes.
• API/view models (FailedMessageView.QueueAddress, QueueAddress.PhysicalAddress) remain string? — non-nullable → nullable assignment is fine.