fix: split FileControlRecord into separate Header/Trailer classes

[ianfnelson]

Description

Realised that the Header and Trailer rows in NBSS Appointment Event files are subtly different - the dates and times refer to the transfer start and end time respectively. Hence they should have different names.
So have replaced the general FileControlRecord class with separate FileHeaderRecord and FileTrailerRecord classes with appropriately named fields.

Context

• No confusion over the meaning of TransferStartTime vs TransferEndTime
• Enables targeted validation: e.g., your HeaderTransferTimeValidator won’t mistakenly run against trailer data
• Self-documenting code: each class maps clearly to the file section it represents
• Can still share parsing logic if the structure is similar, but the model won’t lie

Type of changes

• Refactoring (non-breaking change)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would change existing functionality)
• Bug fix (non-breaking change which fixes an issue)

Checklist

• I am familiar with the contributing guidelines
• I have followed the code style of the project
• I have added tests to cover my changes
• I have updated the documentation accordingly
• This PR is a result of pair or mob programming

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[sonarqubecloud]

Quality Gate failed

Failed conditions
0.0% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud