[DICOM] Constrain cryptoHash output to the target VR's length and character repertoire

[Copilot]

cryptoHash wrote the raw 64-character SHA-256 hex digest into string elements, which violates bounded VRs (e.g. SH allows 16 characters) and made output reconstruction fail with FellowOakDicom.DicomValidationException from DicomValidation.ValidateSH.

{"tag": "PatientTelephoneNumbers", "method": "cryptohash"}   // SH

Before: 2e7acefff0307262cef6f503fa7019257f3f9d47fc987fb2c5a31ae4f4d3c022 (64 chars → validation error)
After: 2e7acefff0307262 (16 chars)

Changes

• CryptoHashProcessor: hashed string values are now conformed to the target VR before being written:
  • truncated to DicomVR.MaximumLength (SH/AE/CS/DS → 16, UI/LO/PN → 64; VRs without a limit such as UC/UT/UR keep the full digest). IS is further capped at 9 digits so the value always fits a 32-bit signed integer.
  • mapped to the VR's character repertoire: digits only for DS/IS/UI (leading zero avoided), uppercase for CS. Output stays deterministic for a given input, key and VR.
• Error reporting: if the generated value is still rejected (fixed-structure VRs AS/DA/DT/TM), an AnonymizerOperationException is raised naming only the tag and VR. The DicomValidationException is deliberately not chained, since it embeds the offending content.
• Tests: per-VR expected values for AE, CS, DS, IS, SH, UI; explicit SH 16-character boundary case; determinism and matchInputStringLength cases; fixed-format VR case asserting the source value does not appear in the message. The DicomResults/Invalid-String-Format.dcm engine fixture was regenerated for the shorter outputs.
• Docs: docs/DICOM-anonymization.md describes the per-VR constraints, notes that truncation reduces collision resistance to the retained characters (64 bits for 16 hex chars), and points to dateShift/redact for AS/DA/DT/TM.

Notes for reviewers

• Truncation is the chosen tradeoff over rejecting bounded VRs outright; the alternative (encoding the digest more densely, e.g. base32) would still be truncated and would break existing output stability.
• Behavior change: cryptoHash output for bounded VRs differs from previous releases, so re-anonymizing the same source data produces different values than before.

• Fixes [DICOM] Ensure crypto hash output respects DICOM VR length limits #267

[azure-pipelines]

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