Clarify request body transforms require existing body

[Asafima]

Improved the documentation for request body transforms by clarifying limitations, common pitfalls, and safe patterns of use.

Request Body Transforms

• Can only modify an existing request body, not add one where none exists.
• Clearly documented:
  • How to safely inject a new request body via custom middleware before YARP
  • Necessary use of IHttpRequestBodyDetectionFeature to ensure ASP.NET Core and YARP recognize the new body
• Provided code examples for adding a new request body in middleware.

Related issues

• Request body transforms only allowed on non-empty requests already yarp#2809
• How to change the request body correctly in middleware yarp#2617

Notes

• The workaround leverages IHttpRequestBodyDetectionFeature, as recommended by @MihaZupan and @Tratcher in How to change the request body correctly in middleware yarp#2617.

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Internal previews

📄 File	🔗 Preview link
aspnetcore/fundamentals/servers/yarp/extensibility-transforms.md	aspnetcore/fundamentals/servers/yarp/extensibility-transforms

[Asafima]

@dotnet-policy-service agree

[MihaZupan]

Thank you