fix: refactor pycon registration validation logic and add unit tests

[seangaaab]

Support Zero Price Registrations on Paid Events & Refactor PyCon Registration Flow

Context

When event.paidEvent is True, the backend previously required a verified transactionId unless a 100% discount code was applied. If an event offered a free ticket tier (price = 0) or temporarily waived ticket fees, the frontend skipped the payment gateway (total === 0), but the backend rejected the registration with a 400 Bad Request due to a missing/unverified transaction ID.

Key Changes

• Server-Side Price Calculation: Dynamically computes the effective total (base_price * (1 - discount)) + sprint_day_price. If the total is <= 0, the registration is marked as free and payment transaction verification is bypassed.
• Reordered Step Execution: Moved ticket type lookup and capacity checks before the payment check to resolve the base ticket price upfront.
• Add-on Protection: Ensures registrations with a free base ticket but a paid Sprint Day (sprintDayPrice > 0) still require payment transaction verification.
• Code Health & Maintainability:
  • Extracted clean private helpers: __resolve_ticket_type_and_price, __resolve_discount, and __validate_payment_transaction.
  • Removed a redundant duplicate database query for existing registrations.
• Testing: Added unit test suite in tests/test_pycon_registration_usecase.py covering 0-dollar tickets, paid tickets, add-on sprint day fees, and 100% discounts.

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[ASPactores]

lgtm