Pulsar: transport-default DLQ/retry precedence + resolve DLQ-sender TODOs (#3186)

[jeremydmiller]

First PR of the Pulsar re-evaluation epic #3176. Fixes #3186.

Resolves the two loose-end TODOs in the Pulsar transport.

Transport-vs-endpoint defaults (PulsarTransport.cs:31 TODO)

The transport-level DeadLetterTopic/RetryLetterTopic are now real defaults that each endpoint inherits unless it sets its own, resolved per-endpoint-override-wins through the new PulsarEndpoint.EffectiveDeadLetterTopic / EffectiveRetryLetterTopic — mirroring the Kafka transport-default + per-endpoint override shape. UsePulsar(...) now returns a PulsarConfiguration exposing DeadLetterQueueing(...) / RetryLetterQueueing(...) to set the defaults. PulsarListener keys native DLQ/retry enablement and the resolved topic names off the effective values, so a transport default actually takes effect.

DLQ sender (PulsarEndpoint.cs:118 TODO) — resolved by decision

The stub looked like missing functionality, but native dead-lettering is already fully handled by PulsarListener (ISupportDeadLetterQueue), which produces to the {topic}-DLQ topic with the native reconsume metadata and retry-letter-topic chaining; WolverineStorage mode uses the durable store.

Implementing a competing endpoint-level TryBuildDeadLetterSender actively regresses this: BufferedReceiver/DurableReceiver then report NativeDeadLetterQueueEnabled, and MessageContext.tryGetDeadLetterQueue prefers the receiver (_channel) over the listener — hijacking the richer native path and dropping the reconsume metadata. This was caught by PulsarNativeReliabilityTests (differential: passes on main, failed with the naive sender impl). So TryBuildDeadLetterSender intentionally defers to the base, documented inline.

DeadLetterStorage now reports Native when a native DLQ is in effect (for monitoring).

Tests

• New unit tests: effective-topic precedence (transport default vs per-endpoint override) for both DLQ and retry, and TryBuildDeadLetterSender is always false (native owned by the listener; WolverineStorage by the durable store).
• Native DLQ end-to-end + exception headers stays covered by the existing PulsarNativeReliabilityTests.

Note: the PulsarNativeReliabilityTests retry-delay assertions are timing-flaky on a loaded/fresh broker (a different one intermittently fails per combined run; each passes in isolation). Not introduced by this PR.

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