Fix missing ParameterDescriptor for form parameters in API descriptions

[rmasciarella]

Summary

HttpChain.fillFormParameters emits ApiParameterDescription instances without populating ParameterDescriptor. ASP.NET Core 10's OpenApiDocumentService.GetFormRequestBody groups form parameters by parameter.ParameterDescriptor.Name with no null guard, so any application that contains a Wolverine HTTP endpoint with a primitive [FromForm] parameter (e.g. [FromForm] string, [FromForm] int, [FromForm] DateTime) returns HTTP 500 from /openapi/v1.json. The 500 takes down the entire OpenAPI document, breaking generated-client tooling (Orval, NSwag, Kiota, etc).

The two sibling form-emit paths in the same file (the FileParameters loop and the IFromFormMetadata loop) already populate ParameterDescriptor with new ParameterDescriptor { Name, ParameterType }. This change extends fillFormParameters to do the same.

_formValueVariables is populated by TryFindOrCreateFormValue, called both directly from primitive [FromForm] parameters and indirectly from FormBindingFrame for the constructor parameters of complex [FromForm] POCOs, so the fix covers both shapes.

Why this scope

Narrow producer-side null-contract fix. ApiParameterDescription.ParameterDescriptor is declared non-nullable in Microsoft.AspNetCore.Mvc.Abstractions; producers should populate it. fillQuerystringParameters and fillKnownHeaderParameters also leave it null, but query/header parameters don't flow through GetFormRequestBody and don't trigger the bug, so they're not changed here.

Repro stack trace

System.NullReferenceException
   at Microsoft.AspNetCore.OpenApi.OpenApiDocumentService.<>c.<GetFormRequestBody>b__33_0(ApiParameterDescription parameter)
   at System.Linq.Lookup`2.Create(...)
   at Microsoft.AspNetCore.OpenApi.OpenApiDocumentService.GetFormRequestBody(...)
   at Microsoft.AspNetCore.OpenApi.OpenApiDocumentService.GetRequestBodyAsync(...)

Regression test

using_form_parameters.form_parameter_descriptions_have_a_parameter_descriptor exercises /form/explicit ([FromForm(Name = "name")] string value), which routes through _formValueVariables → fillFormParameters. Asserts ParameterDescriptor is non-null and carries the right Name + ParameterType.

Fails on main:

should not be null but was
Failed!  - Failed: 1, Passed: 0

Passes after the fix:

Passed!  - Failed: 0, Passed: 1

Test plan

• New test fails on main
• New test passes after fix
• Full using_form_parameters class: 29 passed, 8 pre-existing skips
• Full openapi / swashbuckle / open_api filter: 35 passed, 0 failed

[Copilot]

Pull request overview

Fixes a null-contract issue in Wolverine HTTP’s API description generation that can cause /openapi/v1.json to fail (HTTP 500) on ASP.NET Core 10 when endpoints use primitive [FromForm] parameters.

Changes:

• Populate ApiParameterDescription.ParameterDescriptor for form parameters emitted via _formValueVariables in HttpChain.fillFormParameters.
• Add a regression test asserting ParameterDescriptor is present (and correctly populated) for a primitive [FromForm(Name = ...)] parameter.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/Http/Wolverine.Http/HttpChain.ApiDescription.cs	Ensures form ApiParameterDescription instances include a non-null ParameterDescriptor with Name and ParameterType.
src/Http/Wolverine.Http.Tests/using_form_parameters.cs	Adds a regression test validating ParameterDescriptor is set for primitive [FromForm] parameters used by OpenAPI form request-body generation.

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