fix: parse YAML 1.2 octal scalars (0o prefix) as integers

[raphaelroshan]

Fixes #1251.

YAML 1.2 writes octal as 0o17, but as<int>() failed on it because std::stringstream's octal parsing only understands the C-style 0 prefix.

Normalize 0o/0O to a leading 0 before the stream conversion, so both the 1.1 (0123) and 1.2 (0o123) spellings decode. Hex and decimal are unchanged. As per the discussion in the issue, adding a function NormalizeOctalPrefix that should help to support backward compatibility. Added a test covering octal, signed/unsigned, hex and decimal.

Do let me know if i misinterpreted the behavior required!

[SGSSGene]

@raphaelroshan thank you so much for tackling this issue!

Comments:

1. Could you change the signature to inline std::string NormalizeOctalPrefix(std::string s) I believe this fits the style better.
2. I am not sure if you shuld additionally check that all characters are digits. Otherwise it will accept stuff like 0oxff and convert it to 0xff.
3. Sadly the yaml spec is offline, but ane statements like -0o12 or +0o12 even valid in yaml? (this needs confirmation)

[SGSSGene]

Thanks for this PR, see comment above.

[raphaelroshan]

Thank you @SGSSGene for the review, have updated the signature, added a check to ensure its an octal digit and based on yaml 1.2 - leading +/- signs arent valid so did not add that in the regex check

[SGSSGene]

Thanks for the PR! looks great