[WIP] Fix race condition in PayloadAssembler OnTimedEvent method

[WillButAgain]

Description

Fixes race condition in PayloadAssembler causing message processing failures and dead-lettered RabbitMQ messages.

This PR addresses a critical race condition in the PayloadAssembler.OnTimedEvent method that was causing InvalidOperationException when concurrent threads modified the _payloads dictionary during enumeration. This resulted in:

• RabbitMQ messages failing to be acknowledged
• Messages accumulating in dead-letter queues
• Intermittent export workflow failures

Root Cause:
The OnTimedEvent method iterates over _payloads.Keys while other threads (processing incoming DICOM files via Queue()) can simultaneously add/remove entries. Even though ConcurrentDictionary is thread-safe for individual operations, iterating over its .Keys property while the collection is being modified throws InvalidOperationException.

Solution:
Implemented a snapshot pattern that captures the dictionary keys before iteration, preventing the collection-modified-during-enumeration exception:

• Changed foreach (var key in _payloads.Keys) to use _payloads.Keys.ToList() snapshot
• Added TryGetValue() guard to safely handle keys that may have been removed between snapshot and access
• No locks required—leverages existing ConcurrentDictionary thread safety

Testing:
Added two comprehensive concurrency tests to verify the fix:

• GivenConcurrentAccess_WhenProcessingPayloads_ExpectNoRaceCondition - Tests 100 concurrent queue operations
• GivenHighConcurrentLoad_WhenProcessingPayloads_ExpectNoExceptions - Stress test with 500 concurrent operations

Status

Ready

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• All tests passed locally.
• Documentation comments included/updated.
• User guide updated.
• I have updated the changelog

Impact

• Severity: High
• Affected Component: PayloadAssembler.OnTimedEvent
• Performance Impact: Minimal (small memory allocation for key snapshot)
• Risk: Low - Simple, well-established pattern with no architectural changes

Related Issues

Related to production incidents with:

• 7,928+ messages in dead-letter queues
• Unacknowledged RabbitMQ messages
• InvalidOperationException: Collection was modified; enumeration operation may not execute

See PAYLOAD_ASSEMBLER_RACE_CONDITION_REPORT.md for detailed analysis.