feat: DTOSS-9159 - add file-level validation

[ianfnelson]

Description

Performs file-level validation of NBSS Appointment Event files, covering the following validation rules:

• NBSSAPPT002 - Header is missing
• NBSSAPPT003 - Extract ID is missing
• NBSSAPPT004 - Extract ID is in an invalid format
• NBSSAPPT005 - Record count is missing
• NBSSAPPT006 - Record count is in an invalid format
• NBSSAPPT007 - Trailer is missing
• NBSSAPPT008 - Extract ID does not match value in header
• NBSSAPPT009 - Record count does not match value in header
• NBSSAPPT011 - Record count does not match value in header and trailer

Context

Acts as an anti-corruption layer at the point of ingress to NSP.
Ensures quality of data, guards against coding errors by both parties.

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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Sensitive Information Declaration

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• I confirm that neither PII/PID nor sensitive data are included in this PR and the codebase changes.