Expand Sponsor Module Test Suite

[DiegoPisa2003]

This PR significantly expands the test coverage for the sponsor module, adding comprehensive testing across controllers, models, system tests, and fixtures to ensure robust functionality and maintainability.

Changes Made

Test Infrastructure

• Controller Tests: Added comprehensive test coverage for SponsorsController, Events::SponsorsController, Sponsors::LogosController, and Sponsors::MissingController
• Model Tests: Enhanced Sponsor test with additional test cases and edge cases
• System Tests: Expanded Sponsors test to cover user interactions and UI behavior
• Fixtures: Updated test data in sponsors.yml, events.yml, and event_sponsors.yml to support comprehensive testing

Future Test Coverage (Pending)

The following areas are identified for future testing once dependencies are resolved:

• EventSponsor: Association and relationship testing (pending issue #905 resolution)