OpenAPI: Fix ModelMetadata null reference exception in emitted XMLComment transformers

[desjoerd]

When a route parameter does not have a bound model it would case a null reference exception because the ModelMetadata on the ParameterDescriptor was null. This is fixed by adding a null reference check

Fixes #63757

[Copilot]

Pull Request Overview

This PR fixes a null reference exception in the OpenAPI source generator that occurs when processing route parameters that don't have bound models. The fix adds a null check for ModelMetadata before accessing its properties.

• Adds null safety check for ModelMetadata in XML comment transformer generation
• Updates all generated test snapshots to reflect the fix
• Includes a new test case to verify the fix works correctly

Reviewed Changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated no comments.

File	Description
src/OpenApi/gen/XmlCommentGenerator.Emitter.cs	Adds null check before accessing ModelMetadata properties
src/OpenApi/test/.../OperationTests.Controllers.cs	Adds test case for unused route parameters scenario
Multiple snapshot files	Updates generated code snapshots to include the null check

[dotnet-policy-service]

Thanks for your PR, @@desjoerd. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[desjoerd]

@captainsafia could you review this? Maybe it's still possible to backport this fix into .NET 10. Also when thinking of fixing potential review comments 😊.

[dotnet-policy-service]

Looks like this PR hasn't been active for some time and the codebase could have been changed in the meantime.
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