Add poison message handling for queue deserialization failures

[niemyjski]

Summary

• When a message fails to deserialize during dequeue, it is now abandoned through the normal retry flow instead of crashing the consumer. This gives operators a window to fix transient serializer issues (e.g., missing JsonConverters) before messages are permanently dead-lettered.
• Adds FaultInjectingSerializer test infrastructure (implements ISerializer) for simulating serialization/deserialization failures in tests.
• Adds two new virtual test methods to QueueTestBase: DequeueAsync_WithPoisonMessage_MovesToDeadletterAsync and EnqueueAsync_WithSerializationError_ThrowsAndLeavesQueueEmptyAsync.
• Makes QueueEntry null-safe for poison message entries where deserialization yields null values.
• Documents the poison message handling flow in the queues guide.

Poison Message Flow

Dequeue -> Deserialize fails -> Abandon (attempt incremented) -> Still has retries? -> Re-queued with backoff delay -> Retries exhausted? -> Moved to dead letter queue

Changes

File	Change
docs/guide/queues.md	New section documenting poison message handling
src/Foundatio.TestHarness/Serializer/FaultInjectingSerializer.cs	New test serializer for injecting failures
src/Foundatio.TestHarness/Queue/QueueTestBase.cs	New virtual tests + ISerializer param on GetQueue
src/Foundatio/Queues/QueueEntry.cs	Null-safe Value handling
tests/Foundatio.Tests/Queue/InMemoryQueueTests.cs	Skip new tests (no serialization), remove #region

Provider implementations (separate repos)

Each provider repo has corresponding changes to wire up the poison message handling in their DequeueImplAsync:

• Foundatio.Redis - Catches deserialization errors, creates QueueEntry with null value, calls AbandonAsync
• Foundatio.AzureStorage - Catches deserialization errors, abandons with attempt tracking
• Foundatio.AzureServiceBus - Catches deserialization errors, abandons for retry
• Foundatio.AWS - Catches deserialization errors, abandons for retry

Test plan

• All 1799 tests pass in Foundatio.slnx (1786 succeeded, 13 skipped, 0 failed)
• All provider repos build successfully with ReferenceFoundatioSource=true
• InMemoryQueue tests skip serialization tests (InMemoryQueue does not use serialization)
• Run provider-specific tests against live services (Redis, Azure Storage, Azure Service Bus, SQS)

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