Fix Xcode receipt parsing with strict ASN.1 lengths

[kiwigitops]

Summary

• compute the synthetic value length for Xcode indefinite-length receipts from the value offset instead of the tag offset
• keeps extractTransactionIdFromAppReceipt compatible with the stricter ASN.1 length checks in jsrsasign >= 11.1.2

Closes #417.

Testing

• reproduced the two failing Xcode receipt tests with jsrsasign@11.1.3 before the fix
• npx jest tests/unit-tests/receipt_utility.test.ts --runInBand
• npm run build
• git diff --check

[kiwigitops]

Quick bump on this. The jsrsasign length issue is still reproducible without the offset fix, and I am happy to adjust the receipt parsing change if needed.