Fix OpenAPI request body description uses wrong parameter comment

[BloodShop]

Fixes issue #65805.

When an endpoint has multiple parameters including [FromBody] and injected dependencies (like ILogger or CancellationToken), the request body description was incorrectly using the comment from the LAST parameter instead of the [FromBody] parameter.

Example:

public async Task<Ok<int>> PostSampleData(
    [FromBody] SomeData data,  // Comment: "Sample data provided by the user"
    [FromServices] ILogger logger,
    CancellationToken cancellation  // Comment: "Injected cancellation token"
)

Expected request body description: "Sample data provided by the user."
Actual request body description: "Injected cancellation token."

The bug was in XmlCommentGenerator.Emitter where it loops through all parameters and assigns any unmatched parameter comment to the request body. The last iteration would overwrite previous ones.

Fix: Only use a parameter comment for the request body if it has [FromBody] or is a complex type.

[dotnet-policy-service]

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

[martincostello]

This PR should have test coverage for what's being fixed.

[BloodShop]

Test coverage added. New regression test verifies that when an endpoint has [FromBody] followed by [FromServices] and CancellationToken parameters, the request body description uses the [FromBody] parameter's XML comment, not the last parameter's.

[BloodShop]

Fixed the test failures. The issue was using reflection-based attribute heuristics to identify the request body parameter. Switched to using ApiExplorer's ParameterDescription binding source which is deterministic and already knows which parameter is the body.