fix(multipart): respect MaxBodyBytes limit

[leonklingele]

(huma.Operation).MaxBodyBytes was only enforced for normal request bodies via the
readBody path, but multipart requests take a separate path throug readForm /
(huma.Context).GetMultipartForm() and did not consult op.MaxBodyBytes.

This change makes the MaxBodyBytes value be respected in all paths.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.04%. Comparing base (198225e) to head (b5450f0).

Files with missing lines	Patch %	Lines
huma.go	66.66%	5 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1094      +/-   ##
==========================================
- Coverage   93.20%   93.04%   -0.16%     
==========================================
  Files          23       23              
  Lines        4988     5003      +15     
==========================================
+ Hits         4649     4655       +6     
- Misses        272      278       +6     
- Partials       67       70       +3     

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[Copilot]

Pull request overview

This PR extends request body size limiting so that huma.Operation.MaxBodyBytes is enforced for multipart requests (which previously bypassed the readBody path), and adds test coverage to prevent regressions.

Changes:

• Pass op.MaxBodyBytes into the multipart parsing path (readForm) so multipart uploads are subject to the same limit.
• Implement multipart parsing via a limited reader when MaxBodyBytes > 0 to detect oversize bodies.
• Add a unit test ensuring oversized multipart requests return HTTP 413 and do not invoke the handler.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
huma.go	Routes multipart parsing through a new readForm(ctx, maxBytes) implementation to enforce MaxBodyBytes.
huma_test.go	Adds a test verifying multipart requests exceeding MaxBodyBytes return 413 and do not call the handler.

Suppressed comments (2)

huma.go:1029

• When readForm returns a StatusError (e.g. 413 for MaxBodyBytes), this path appends it to res.Errors and later wraps it with WriteErr(..., "validation failed", ...), so clients get a 413 with an unrelated top-level detail message. Consider short-circuiting and writing StatusError responses directly here (similar to the handler error path).

This issue also appears on line 2075 of the same file.

				if err != nil {
					res.Errors = append(res.Errors, err)
				} else {

huma.go:2077

• On multipart parse errors (ReadForm returning a non-nil form + err), the current code returns the form without cleaning up any temporary files created during parsing, and the caller ignores form on error. This can leak temp files on malformed/aborted uploads.

	if err != nil {
		return form, &ErrorDetail{Location: "body", Message: "cannot read multipart form: " + err.Error()}
	}

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