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