Fix NuGet package upload error handling

[silverwind]

Wrap zip.NewReader errors in NuGet ParsePackageMetaData and ExtractPortablePdb as ErrInvalidArgument so invalid packages return HTTP 400 (Bad Request) instead of 500 (Internal Server Error).

Add integration test for multipart/form-data NuGet upload path (used by dotnet nuget push) which was previously untested.

Note: I was unable to reproduce the reported error with dotnet nuget push (.NET 10) against a local Gitea instance — the upload succeeded. The reporter's issue may be environment-specific (reverse proxy configuration, .NET version, etc). These changes improve error reporting for invalid uploads regardless.

Ref #36932

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This PR was written with the help of Claude Opus 4.6

[Copilot]

Pull request overview

Improves NuGet package upload robustness by preventing panics on malformed multipart requests and ensuring invalid ZIP uploads are treated as client errors (HTTP 400), plus adds an integration test covering multipart/form-data uploads.

Changes:

• Guard Context.UploadStream() against a nil Req.MultipartForm after ParseMultipartForm, avoiding a potential nil dereference and removing a dead application/x-www-form-urlencoded branch.
• Wrap zip.NewReader failures in NuGet metadata/symbol parsing as util.ErrInvalidArgument to return HTTP 400 for invalid packages instead of HTTP 500.
• Add an integration test that uploads a NuGet package using multipart/form-data.

Reviewed changes

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

File	Description
tests/integration/api_packages_nuget_test.go	Adds an integration test for multipart/form-data NuGet uploads.
services/context/context_request.go	Hardens UploadStream() to avoid nil dereference on multipart parsing and removes dead code.
modules/packages/nuget/metadata.go	Maps invalid ZIP reader errors to ErrInvalidArgument (HTTP 400) for .nupkg parsing.
modules/packages/nuget/symbol_extractor.go	Maps invalid ZIP reader errors to ErrInvalidArgument (HTTP 400) for .snupkg parsing.

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[wxiaoguang]

Updated to different messages to help distinguishing different failure cases