Investigate skipped merchant schedule validation tests

[StuartFerguson]

Background

Two merchant schedule page tests were temporarily skipped to unblock CI after repeated failures in GitHub Actions:

• MerchantSchedule_SaveSelectedYear_InvalidNovemberDate_ShowsErrorAndDoesNotSave
• MerchantSchedule_SaveSelectedYear_NonLeapYearFebruary_Rejects29th

Goal

Identify why these tests fail in CI and restore them as active assertions without using [Fact(Skip = ...)].

Tasks

• Reproduce the failures in a controlled environment
• Compare local vs CI behavior for bUnit rendering and async component state updates
• Verify whether the component updates errorMessage and/or rendered markup consistently after invalid save attempts
• Check for timing/order issues around LoadYear, input change handling, and SaveScheduleAsync
• Replace brittle assertions with stable ones if needed
• Re-enable both tests once stable

Definition of done

• Both tests run without skip attributes
• CI passes consistently
• Any root cause and fix are documented in the PR or issue