[DICOM] Remove Int64 overflow path in length-matched crypto hash output

[Copilot]

MatchInputStringLength=true could throw OverflowException for numeric inputs with 19+ digits because the length-preserving path depended on Int64 bounds. This change removes that failure mode while keeping deterministic, numeric-only, exact-length output behavior.

• Length-preserving hash generation

  • Reworked GenerateOutputOfSameLength to avoid Int64-bounded range construction for long inputs.
  • Kept existing behavior for shorter inputs (<=18 digits) where practical.
  • Added a deterministic BigInteger-based mapping for longer lengths (>18) to preserve fixed-length numeric output without overflow.

• Deterministic long-length expansion

  • Added deterministic hash-byte expansion for large numeric ranges and mapped into the valid digit-space range.
  • Documented the entropy expansion choice and modulo-range mapping behavior in code.

• Boundary coverage for regression

  • Added/updated unit coverage for 1-, 18-, 19-, and longer-character inputs (40 chars).
  • Assertions focus on: same-length output, numeric-only output, and determinism for identical input/key.

// Long-length path no longer parses "999...9" into Int64.
BigInteger lowerBound = BigInteger.Pow(10, input.Length - 1);
BigInteger range = lowerBound * 9;
byte[] expandedHash = ExpandHash(hash, requiredBytes);
BigInteger value = new BigInteger(expandedHash, isUnsigned: true, isBigEndian: true);

return (lowerBound + (value % range)).ToString(CultureInfo.InvariantCulture);

• Fixes [DICOM] Prevent overflow when matching crypto hash output length #265

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