Rework MT535 parser

[robocode13]

Replace the old MT535 parser with a new implementation and update tests to ensure correct parsing of various input formats.

[robocode13]

@Nils-Fischer I re-implemented the parser you originally contributed because i had problems with parsing holdings for two different banks (DKB and Baader). Some of the original test cases failed because they seem to use some non-standard formats. Were these test cases based on real responses from a bank or were they just synthetic (maybe AI generated ?). Of course i want to make sure that the new parser won't break anything that was working before. If you have some real bank responses, maybe you could provide them, of course replacing all sensitive/private data first but keeping the structure. Thanks!