reject requests with more than one Host header field

[sahvx655-wq]

basic_parser rejects a duplicate Content-Length but quietly accepts two Host header fields, which RFC 7230 5.4 forbids because a front-end and the back-end can then resolve the request to different authorities, a routing and cache-poisoning vector.

1. do_field acts on Connection, Content-Length, Transfer-Encoding and Upgrade but never looked at Host, so a second Host was merged like any ordinary field.
2. track it with a flagHost bit and fail the request with error::multiple_host on the second occurrence, request-side only so response parsing is unchanged.

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.25%. Comparing base (736daa0) to head (3b69d98).

Files with missing lines	Patch %	Lines
include/boost/beast/http/impl/error.ipp	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3097      +/-   ##
===========================================
- Coverage    93.28%   93.25%   -0.04%     
===========================================
  Files          177      177              
  Lines        13750    13756       +6     
===========================================
+ Hits         12827    12828       +1     
- Misses         923      928       +5     

Files with missing lines	Coverage Δ
include/boost/beast/http/basic_parser.hpp	94.11% <ø> (ø)
include/boost/beast/http/impl/basic_parser.ipp	97.24% <100.00%> (+0.03%)	⬆️
include/boost/beast/http/impl/error.ipp	93.87% <0.00%> (-1.96%)	⬇️

... and 1 file with indirect coverage changes

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