Add sending failure policies for outgoing message error handling

[jeremydmiller]

Summary

• Implements configurable failure handling policies for outgoing message send failures (Issue Support error handling in the durable outbox to allow messages to be moved to DLQ or discarded if rejected by the transport/broker #1686)
• When a message fails to send to an external transport, users can now configure policies to discard, move to dead letter queue, retry with backoff, latch the sender, or take custom actions
• Unmatched exceptions fall through to existing retry/circuit-breaker behavior, preserving full backwards compatibility
• Adds MessageTooLargeException with detection in Azure Service Bus sender for messages that can never fit in any batch

User-facing API

// Global: discard messages that are too large
opts.SendingFailure.OnException<MessageTooLargeException>().Discard();

// Global: move to dead letter after 3 retries
opts.SendingFailure.OnException<ServiceBusException>()
    .RetryTimes(3).Then.MoveToErrorQueue();

// Per-endpoint override
opts.PublishAllMessages().ToAzureServiceBusTopic("orders")
    .ConfigureSending(s => s.OnException<ServiceBusException>().Discard());

Key changes

• SendingFailurePolicies class reusing existing IWithFailurePolicies/FailureRuleCollection infrastructure
• SendingEnvelopeLifecycle adapter enabling existing IContinuation implementations to work on outgoing envelopes
• LatchSenderContinuation to pause sending agents (mirrors PauseListenerContinuation)
• Envelope.SendAttempts for independent send attempt tracking
• Integration into SendingAgent, DurableSendingAgent, BufferedSendingAgent, and InlineSendingAgent
• EndpointCollection resolves combined global + per-endpoint policies during agent construction

Test plan

• Core Wolverine project builds with 0 errors
• Azure Service Bus project builds with 0 errors
• Slim solution builds with 0 errors across all projects
• 1000/1003 CoreTests pass (3 pre-existing failures unrelated to this change)
• Azure Service Bus integration tests (requires emulator infrastructure)

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