Add bounds checks tests for MT940 structured parsing

[larsdecker]

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[Copilot]

Pull request overview

This pull request adds security bounds checks to three MT940 parsing functions to prevent processing of excessively long input strings that could impact performance or security. The changes include input length validation with appropriate error handling and comprehensive test coverage for these new bounds checks.

• Added length validation checks (1000 chars for parsePaymentReferenceDate and parsePaymentReferenceTan, 10000 chars for assemblePaymentReference)
• Implemented error throwing when input exceeds defined limits
• Added comprehensive test suites covering boundary conditions, error cases, and valid input scenarios

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 15 comments.

File	Description
packages/fints/src/mt940-86-structured.ts	Added bounds check validation to three parsing functions with error handling
packages/fints/src/tests/test-mt940-86-structured.ts	Added three new test suites with 9 tests total covering bounds validation, edge cases, and normal operation

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