Fix deadlocks in async-to-sync bridge by properly managing synchronization context

Description

• Async method calls through the synchronous RunSync bridge were causing deadlocks due to improper synchronization context management
• Continuations were being queued to the thread pool instead of the custom synchronization context, breaking the synchronous execution model
• The CustomSynchronizationContext implementation needed alignment with the proven Rebus library implementation pattern

Deliverables

• Context Management: Move synchronization context setup/teardown from RunSync into CustomSynchronizationContext.Run() for cleaner separation of concerns
• Continuation Handling: Remove ConfigureAwait(false) from PostCallback to ensure all continuations execute on the custom context
• API Consistency: Add ConfigureAwait(false) to RunSync method to maintain consistent async patterns
• Code Clarity: Add comments explaining the context behavior and why ConfigureAwait is not used in PostCallback

Start by locating RunSync and CustomSynchronizationContext, then compare their context setup, teardown, and PostCallback behavior with the proven Rebus implementation pattern. The work is done when context ownership is moved into CustomSynchronizationContext.Run(), continuations remain on the custom context, RunSync uses ConfigureAwait(false), and the context behavior is documented.