Support chunk extensions in chunked decoder

[kevinresol]

The chunked decoder parsed the whole chunk-size line as a hex number, so a valid chunk header carrying a chunk extension (e.g. 3;foo=bar, RFC 7230 §4.1.1) was fed to Std.parseInt as 0x3;foo=bar and failed to decode.

• Strip the optional chunk extension (everything from the first ;) before parsing the hex size.
• Reject invalid/empty sizes with a parse error instead of silently producing a bad state.

Added TestChunked decode variants covering 3;foo=bar and extensions on the terminating 0 chunk.

Made with Cursor

[kevinresol]

Addressed review: switched Std.string(v) to v.toString() in the chunked decoder. Also rebased onto latest master to keep the PR clean.