feat: add ExceptionHandlingBehavior to centralize error handling

[Mehrdad-Sohayl]

Summary

Adds ExceptionHandlingBehavior MediatR pipeline behavior to centralize exception handling in command handlers.

Changes

• New: src/Wallet.Application/Common/Behaviors/ExceptionHandlingBehavior.cs - Pipeline behavior that intercepts:
  • FluentValidation errors → Result.Failure
  • Business rule violations (InvalidOperationException with specific messages) → Result.Failure
  • Concurrency conflicts (DbUpdateConcurrencyException) → Result.Failure with standard message
  • Re-throws if response type isn't Result<T>
• Modified: src/Wallet.Application/DependencyInjection.cs - Added DI registration

Benefits

• Eliminates duplicate try/catch blocks in 8+ command handlers (TopUpWallet, FastPay, Reserve, ConfirmReservation, CancelReservation, Refund, ConsumePromo, AddPromoGrant)
• Centralized error handling logic
• Consistent error responses
• Cleaner handlers focused on business logic

Testing

• All 143 existing tests pass (Unit: 111, Integration: 11, Acceptance: 11, Property: 5, Architecture: 5)
• Build succeeds with 0 warnings/errors