test: add Application handler tests for all missing features
New test files (4 tests each — 2 success + 2 failure):
- MergePdfsCommandHandlerTests: TwoStreams, WithPageRanges, BadRequest, GenericException
- AddAnnotationHandlerTests: TextMarkup, FreeText, BadRequest (page OOB), GenericException
- AddStampHandlerTests: TextStamp, PredefinedStamp, BadRequest (opacity), GenericException
- ExtractPdfAttachmentsHandlerTests: ReturnsZip, NotFoundException, BadRequest, GenericException
- ConvertPdfAHandlerTests (ConvertTo + ConvertFrom):
ConvertTo: ValidLevel, NotImplemented, BadRequest
ConvertFrom: ValidPdfA, BadRequest, GenericException
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