Fix issues 2139, 2217 handling Auto delimiter in fromDecimal function

[alleria173]

Description
fromDecimal in src/core/lib/Decimal.mjs has always accepted delim="Auto" as its default, but never implemented it. It passed "Auto" directly to Utils.charRep(), which has no entry for that string and returns undefined. Calling str.split(undefined) in JavaScript returns the whole string as a single-element array, so the input was never actually split into individual values.

This caused Utils.convertToByteString and Utils.convertToByteArray (called with type="decimal") to misparse their input. The most visible symptom was that the HMAC operation produced different results for a Latin1 key of AA vs a Decimal key of 65 65, even though both represent the same two bytes.

The fix adds an explicit "Auto" branch that splits on the regex /[^\d-]+/ — any run of characters that are not digits or minus signs — and filters out empty strings. This mirrors the approach already used by fromHex, which has its own "Auto" regex branch (/[^a-f\d]|0x/gi). Explicit named delimiters continue to go through Utils.charRep as before.

Existing Issue
Fixes #2139 and fixes #2217.

Screenshots
N/A — no visual changes.

AI disclosure
GitHub Copilot (Claude Sonnet 4.6) was used to assist in diagnosing the root cause and drafting the fix. The code has been reviewed and verified manually.

Test Coverage
Three HMAC regression tests added to tests/operations/tests/Hash.mjs:

• HMAC: Decimal key space-delimited matches Latin1 key — verifies "65 65" (the previously broken case) produces the correct digest
• HMAC: Latin1 key matches space-delimited Decimal key — the known-good Latin1 baseline ("AA") producing the same digest
• HMAC: Decimal key comma-delimited matches Latin1 key — verifies Auto also handles comma-separated input ("65,65")

[alleria173]

Hi there. Let me know if you need any more commentary/tests to support this pull request.

[GCHQDeveloper581]

Thanks for your contribution. These issues have now been resolved by the merge of #2270.

The HMAC tests would still be useful though - would you care to submit these as a separate stand-alone PR?

[alleria173]

@GCHQDeveloper581 No problem, I've put the HMAC tests into their own PR #2680 👍