fix: OpenAPI request body description uses correct parameter comment

[BloodShop]

Summary

This PR fixes issue #65805 where OpenAPI request body descriptions were incorrectly using the last parameter's XML comment instead of the [FromBody] parameter's comment.

Problem

When an endpoint had multiple parameters (e.g., [FromBody] parameter followed by [FromServices] or CancellationToken), the OpenAPI generator would incorrectly assign the last parameter's XML comment to the request body description instead of using the comment from the parameter that actually represents the request body.

Root Cause

The original loop that assigns parameter comments would iterate through all parameters and assign any unmatched parameter's comment to the request body. This caused the last iteration (usually an injected dependency like CancellationToken) to overwrite the correct [FromBody] parameter's comment.

Solution

Updated the logic to:

1. Use ParameterDescription.Source from ApiExplorer to correctly identify which parameter represents the request body
2. Only assign a parameter's comment to the request body if it has a binding source that indicates it's for request body (Body, FormFile, or Form)
3. Prevent overwriting requestBody.Description if already set, ensuring the correct comment is preserved

Files Modified

• src/OpenApi/gen/XmlCommentGenerator.Emitter.cs - Core fix to parameter comment assignment logic
• src/OpenApi/test/Microsoft.AspNetCore.OpenApi.SourceGenerators.Tests/OperationTests.MinimalApis.cs - Added regression test

Benefits

• ✅ Request body descriptions now correctly use the [FromBody] parameter's XML comment
• ✅ No more confusion with injected dependencies overwriting correct comments
• ✅ Robust identification using actual binding source instead of heuristics
• ✅ Comprehensive test coverage to prevent regression

Testing

Added regression test that verifies when an endpoint has [FromBody] followed by [FromServices] and CancellationToken parameters, the request body description uses the [FromBody] parameter's comment ("Sample data provided by the user") and not the last parameter's comment ("Injected cancellation token").

This is a clean version that contains only the OpenAPI fix without the infrastructure changes that were causing test failures in the previous PR.

Fixes #65805

[martincostello]

Appears to be an exact duplicate of #65827.

[BloodShop]

Closing duplicate PR. Updated the original PR #65827 with clean commits instead. Sorry for the confusion!