validate quoted-string in parse_chunk_extensions

[sahvx655-wq]

Quoted chunk-ext values skip character validation

While going over the chunked request path I noticed parse_chunk_extensions walks a quoted chunk-ext value looking only for the closing " and a backslash escape, without ever checking the octets in between. The rfc7230 quoted-string grammar only permits qdtext inside the quotes and a qpchar after a backslash, yet the loop accepts anything, so NUL, other control characters, DEL and a bare LF all parse cleanly. param_iter already validates header-parameter quoted-strings with is_qdchar/is_qpchar; the chunk parser just never got the same treatment.

The bare LF is what bothers me here. find_eol only recognises CRLF as the chunk-line terminator, so Beast keeps 1;a="<LF>" together as a single line, whereas a peer that accepts a lone LF as a line ending would cut the chunk-size line at the LF and disagree about where the next chunk begins, which is the usual shape of a chunked desync. The added regression case in basic_parser.cpp fails on the unpatched parser and passes after; valid quoted extensions still parse.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.28%. Comparing base (5c484a3) to head (eec2ee1).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3102      +/-   ##
===========================================
- Coverage    93.31%   93.28%   -0.03%     
===========================================
  Files          177      177              
  Lines        13754    13760       +6     
===========================================
+ Hits         12834    12836       +2     
- Misses         920      924       +4     

Files with missing lines	Coverage Δ
include/boost/beast/http/detail/basic_parser.ipp	88.49% <100.00%> (+0.19%)	⬆️

... and 1 file with indirect coverage changes

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