Fix file upload FormatException for parameterized Content-Type

[michaelstaib]

Fixes #9922

[Copilot]

Pull request overview

This PR aims to prevent a FormatException during multipart file uploads when the provided file Content-Type includes parameters (for example text/plain; charset=utf-8), and adds regression coverage in both Fusion and Transport HTTP client tests.

Changes:

• Adds a Fusion snapshot test covering forwarding of parameterized file Content-Type values.
• Extends GraphQLHttpClientTests to include a parameterized Content-Type case.
• Updates DefaultGraphQLHttpClient multipart file part header handling for Content-Type.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/HotChocolate/Fusion/test/Fusion.AspNetCore.Tests/FileUploadTests.cs	Adds a Fusion regression test for parameterized file Content-Type.
src/HotChocolate/Fusion/test/Fusion.AspNetCore.Tests/__snapshots__/FileUploadTests.Upload_Single_File_Should_Forward_Parameterized_ContentType.yaml	Adds snapshot asserting the multipart part includes Content-Type: text/plain; charset=utf-8 and the server sees it.
src/HotChocolate/AspNetCore/test/Transport.Http.Tests/GraphQLHttpClientTests.cs	Adds theory input for parameterized Content-Type in file upload test coverage.
src/HotChocolate/AspNetCore/src/Transport.Http/DefaultGraphQLHttpClient.cs	Changes how multipart file part Content-Type is set (currently via TryAddWithoutValidation).

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[github-code-quality]

Code Coverage Overview

Languages: C#

C# / code-coverage/dotnet

The overall coverage in the mst/fix-9922 branch remains at 49%, unchanged from the main branch.

Show a code coverage summary of the most impacted files.

File	main ef8b6f3	mst/fix-9922 1b4702c	+/-
/home/runner/wo...tionExecutor.cs	70%	56%	-14%
/home/runner/wo...ageProcessor.cs	100%	89%	-11%
/home/runner/wo...gerMessage.g.cs	19%	11%	-8%
/home/runner/wo...Subscription.cs	92%	90%	-2%
/home/runner/wo...geDispatcher.cs	77%	75%	-2%
/home/runner/wo...boxProcessor.cs	74%	72%	-2%
/home/runner/wo...PromiseCache.cs	78%	81%	+3%
/home/runner/wo...eiveEndpoint.cs	91%	94%	+3%
/home/runner/wo...qlConnection.cs	89%	92%	+3%
/home/runner/wo...ntMiddleware.cs	69%	85%	+16%

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