feat(multipart): add multipart JSON field explode support

[lsdch]

Follow up to #1060, which adds support for decoding multipart fields with contentType:"application/json".

Until now, array fields were only handled as a single JSON value. This limited interoperability with OpenAPI clients, which need to follow the encoding.explode rules from OpenAPI spec when serializing multipart requests.

For example, an array field can be represented in two valid ways.

With explode: false:

Content-Disposition: form-data; name="tags"
Content-Type: application/json

["tag1", "tag2"]

With explode: true (the default for multipart requests):

Content-Disposition: form-data; name="tags"
Content-Type: application/json

"tag1"

Content-Disposition: form-data; name="tags"
Content-Type: application/json

"tag2"

Huma should accept both representations according to the OpenAPI contract.

This PR:

• adds support for the OpenAPI encoding.explode setting on multipart form fields (only effective for array properties)
• preserves multipart encoding metadata in the generated OpenAPI document
• supports decoding JSON arrays with both explode=true and explode=false

For example:

type Input struct {
	RawBody huma.MultipartFormFiles[struct {
		Tags   []string `form:"tags" contentType:"application/json"` // explode=true by default
		Numbers []int   `form:"numbers" contentType:"application/json" explode:"false"`
		Value  string   `form:"value" contentType:"application/json"`
	}]
}

This change assumes that clients respect the OpenAPI multipart encoding contract. Fields declared with contentType:"application/json" must always contain valid JSON.

[codecov]

Codecov Report

❌ Patch coverage is 90.47619% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.18%. Comparing base (9d0a320) to head (6cf485d).

Files with missing lines	Patch %	Lines
huma.go	87.50%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1093      +/-   ##
==========================================
- Coverage   93.20%   93.18%   -0.02%     
==========================================
  Files          23       23              
  Lines        4988     5022      +34     
==========================================
+ Hits         4649     4680      +31     
- Misses        272      274       +2     
- Partials       67       68       +1     

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[Copilot]

Pull request overview

Adds support for OpenAPI encoding.explode when decoding multipart form fields tagged with contentType:"application/json", enabling array JSON fields to be represented either as a single JSON array part (explode=false) or as multiple JSON item parts (explode=true, default).

Changes:

• Add explode struct-tag support for multipart JSON array decoding (explode=true multi-part items; explode=false single JSON array).
• Preserve explode metadata in the generated OpenAPI multipart encoding object.
• Update tests and docs to reflect exploded multipart JSON array behavior.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
schema.go	Introduces maybeBoolTag to support optional boolean tags (needed for OpenAPI encoding).
huma.go	Applies explode to form fields and updates multipart JSON decoding to support exploded arrays.
huma_test.go	Updates multipart JSON array test payload to use exploded representation for tags and non-exploded for numbers.
formdata.go	Emits Encoding.Explode into generated OpenAPI multipart encoding (when tag is set).
docs/docs/features/request-inputs.md	Documents explode support for multipart JSON fields with examples for both modes.

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