feat(client): prefer multipart and support url-encoded file uploads

[fredbi]

When an operation advertises both multipart/form-data and application/x-www-form-urlencoded in consumes, the runtime now prefers multipart regardless of the order produced by codegen. This avoids silently falling into the URL-encoded path (which doesn't stream and drops per-file Content-Type) just because it happened to be listed first.

When the caller does explicitly select application/x-www-form-urlencoded (e.g. via go-swagger's WithContentType option from go-swagger/go-swagger#3042), file fields are now properly URL-encoded as form values instead of producing a multipart body advertised under a urlencoded Content-Type. Removed the unreachable / non-conformant branch in mangleContentType that emitted "application/x-www-form-urlencoded; boundary=...".

Fixes #286

Change type

Please select: 🆕 New feature or enhancement|🔧 Bug fix'|📃 Documentation update

Short description

Fixes

Full description

Checklist

• I have signed all my commits with my name and email (see DCO. This does not require a PGP-signed commit
• I have rebased and squashed my work, so only one commit remains
• I have added tests to cover my changes.
• I have properly enriched go doc comments in code.
• I have properly documented any breaking change.

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.67%. Comparing base (1114423) to head (5436226).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
client/request.go	73.33%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #428      +/-   ##
==========================================
- Coverage   81.71%   81.67%   -0.05%     
==========================================
  Files          50       50              
  Lines        3397     3411      +14     
==========================================
+ Hits         2776     2786      +10     
- Misses        508      510       +2     
- Partials      113      115       +2     

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