fix: preserve 0x prefix when coercing hex string with byteAligned

[JSap0914]

Bug

string.hex({ byteAligned: true }) combined with prefix: 'optional' or prefix: true incorrectly coerces odd-length hex strings. The coerce phase prepends '0' before the entire value — including the 0x/0X prefix — producing an invalid hex string that then fails the string.hex regex check:

Joi.string().hex({ byteAligned: true, prefix: 'optional' }).validate('0xABC');
// Before fix → { value: '00xABC', error: '"value" must only contain hexadecimal characters' }
// Expected   → { value: '0x0ABC' }

Fix

When the value starts with a 0x/0X prefix, insert the padding zero after the two-character prefix instead of before the whole string. Values without a prefix are unaffected.

Files changed: lib/types/string.js (1 line → 5 lines), test/types/string.js (18 lines added)

Verification

npx lab test/types/string.js --reporter console
# 272 tests complete (271 existing + 1 new)

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