Expose per-endpoint dead-letter destination as a queryable contract (#3104)

[jeremydmiller]

Closes #3104.

Summary

There was no transport-agnostic way to ask "where do this endpoint's dead letters go?" — it was encoded in per-transport config (DeadLetterQueueName, DeadLetterQueueMode, the ASB $DeadLetterQueue sub-queue, …). This adds a uniform, queryable contract so a monitor like CritterWatch can detect endpoints that dead-letter natively without recovery to durable storage and recommend the fix — without leaking transport-specific knowledge.

The contract

A transport-agnostic enum:

public enum DeadLetterStorageMode { Durable, Native, NativeWithRecovery }

exposed two ways:

• Endpoint.DeadLetterStorage (virtual, default Durable) on the endpoint model.
• EndpointDescriptor.DeadLetterStorage as a first-class typed field on the diagnostic descriptor surface monitoring tools read (the Promote Mode + IsListener to typed fields on EndpointDescriptor; drop Wolverine.Http capability's redundant Sets.Routes #3009 pattern; the reflected Properties row is dropped so it isn't double-shipped).

Per-transport reporting

Transport	Reports
RabbitMQ	Native (DLX) · NativeWithRecovery when EnableDeadLetterQueueRecovery() is set · Durable for WolverineStorage mode
Amazon SQS	Native when a DLQ is configured · Durable when disabled
Azure Service Bus	Native (managed DLQ queue / native sub-queue) · Durable when disabled
Redis / Kafka / GCP Pub/Sub	Native when a native DLQ is configured · Durable otherwise
Local / others	Durable

Only RabbitMQ can report NativeWithRecovery today, since it's the only transport with native→durable recovery on main. Once #3103 (recovery for SQS/Azure Service Bus) merges, a tiny follow-up can promote those transports to NativeWithRecovery when recovery is enabled. The enum value is in place for it.

Documentation

New "Introspecting an Endpoint's Dead Letter Destination" section on the Dead Letter Storage page, with the enum semantics table and how to read the contract.

Tests

• Core: descriptor defaults to Durable and doesn't duplicate the value in Properties.
• Per-transport contract tests for SQS, Azure Service Bus, and RabbitMQ (infrastructure-free — direct endpoint construction).

Verification

• Full wolverine.slnx Release build clean (0 warnings / 0 errors).
• All new tests green (3 core + 3 SQS + 3 ASB + 3 RabbitMQ).

Pairs with

#3103 — together a monitor can (a) detect un-bridged native DLQs via this contract and (b) point users at the one-call EnableDeadLetterQueueRecovery() fix.

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