Add support for multipart/mixed content type

[hhaensel]

This PR is a port of the older #1100 with the help of Claude 4.5
It fixes #1101.

This commit adds comprehensive support for multipart/mixed content, commonly used for batch API requests (e.g., SharePoint, GraphQL).

Core functionality:

• Added type field to Form struct to support :formdata and :mixed
• Form constructor accepts type parameter (defaults to :formdata)
• Form(Vector{Multipart}) constructor for multipart/mixed
• Updated content_type() to return correct MIME type based on form type

Parsing enhancements:

• Modified parsing to optionally skip Content-Disposition headers (not required for multipart/mixed per RFC2046)
• Added parse_multipart() as generic parser for any multipart type
• Added parse_multipart_mixed() convenience function
• Support for type filtering in parsing functions

API additions:

• Batch() convenience function for semantic batch request creation
• Exported: parse_multipart, parse_multipart_mixed, Batch

Testing:

• Comprehensive test suite in test/http_multipart_mixed.jl
• 41 tests covering creation, parsing, round-trip, and edge cases

The implementation is adapted from the original work in commit 573fb62 but updated to work with HTTP v2 architecture where multipart functionality is consolidated in http_forms.jl.

[codecov]

Codecov Report

❌ Patch coverage is 97.18310% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.33%. Comparing base (51f101a) to head (4b9ead5).
⚠️ Report is 25 commits behind head on master.

Files with missing lines	Patch %	Lines
src/http_forms.jl	97.18%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1291      +/-   ##
==========================================
+ Coverage   84.60%   87.33%   +2.73%     
==========================================
  Files          28       29       +1     
  Lines       10787    11356     +569     
==========================================
+ Hits         9126     9918     +792     
+ Misses       1661     1438     -223     

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[hhaensel]

@quinnj Just found some possible improvements, please don't merge yet

[hhaensel]

@quinnj just review as is! The idea I had was not so good...

[hhaensel]

@quinnj just need feedback on my two open conversations.
I tend to revert the @assert change, it seems that @assert is quite common in HTTP

[hhaensel]

Don't merge yet, I'm still adding an example ...

[hhaensel]

I've added a multibatch example including server. It turned out that the parsing was needed for both Requests and Responses, so I decided to unify http_forms.jl accordingly. Here's what I did in detail

• Refactor to use Union{Request,Response} pattern (matches http_core.jl style)
• Rename helper _request_body_bytes() to `_message_body_bytes()
• Add comprehensive Response parsing tests
• Include batch server example demonstrating multipart/mixed usage
• Update docstrings with jldoctest examples

This is all fully backward compatible with existing Request API

[hhaensel]

@quinnj ready for final review :-)

[hhaensel]

Found a bug in HTTP 2.3.0, it references VERSION in HTTP's scope for the public declarations. I fixed it here and also filed #1313