Lock in custom-serializer-on-send guarantee (CritterWatch#261 H1 analysis)

[jeremydmiller]

Pre-release verification for the H1 hypothesis in CritterWatch#261: "Wolverine SQS endpoint not honoring the rule-level DefaultSerializer."

Conclusion: the framework honors custom serializers

Traced the send path end to end — the mechanism is sound, no production bug:

1. x.To(uri).DefaultSerializer(custom) → SubscriberConfiguration delays e.DefaultSerializer = custom, whose setter RegisterSerializer(custom)s it (keyed by custom.ContentType) and stores it on the Endpoint.
2. Endpoint.Compile() uses DefaultSerializer ??= runtime.Options.DefaultSerializer — so an endpoint-level custom serializer is not overwritten by the global STJ default.
3. MessageRoute captures Serializer = endpoint.DefaultSerializer, and CreateForSending sets envelope.Serializer = Serializer + envelope.ContentType = Serializer.ContentType. The sending agent then writes the wire bytes via envelope.Serializer.Write(envelope).

So a custom IMessageSerializer (e.g. a Brotli-compressing one) is the serializer that produces the wire payload — provided the rule lands on the endpoint the message routes to.

On the URI-mismatch sub-hypothesis (H2)

AmazonSqsTransport.findEndpointByUri falls back to Queues[uri.OriginalString.Split("//")[1].TrimEnd('/')], so sqs://critterwatch and sqs://critterwatch/ resolve to the same queue endpoint — the trailing-slash difference seen in the logs does not split the serializer onto a phantom endpoint at the framework level.

The #261 symptom is therefore most likely on the CritterWatch-extension / host-config side (H3 host-level DefaultSerializer override after AddCritterWatchMonitoring, H4 Wolverine.CritterWatch 0.7.0 version regression, or H5 strip not running) — the diagnostic logging in that comment will localize it. Not a Wolverine framework defect.

What this PR adds

The existing serialization_configuration tests only assert the serializer is attached to the endpoint. This adds a send-path regression test that routes a message through runtime.RoutingFor(...).RouteForSend(...) and asserts:

• the outgoing envelope for the overridden endpoint carries the custom serializer + its content type,
• envelope.Serializer.Write(envelope) is what produces the payload,
• a sibling endpoint with no override stays on the global SystemTextJsonSerializer.

No production code change — this locks in the guarantee so a future regression (e.g. a Compile() ordering change that lets the global default clobber an endpoint override) is caught. serialization_configuration suite green (8/8).

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