Fix OpenAPI schema generation for FileContentResult to use binary format

[shethaadit]

Fix OpenAPI schema generation for FileContentResult to use binary format

Summary of the changes (Less than 80 chars)

Description

This PR fixes #63172 by ensuring that FileContentResult is correctly represented as a binary file response in the generated OpenAPI schema instead of being treated as a JSON object reference.

Changes

• Added FileContentResult to the list of types that should be represented as binary data in the OpenAPI schema
• Modified the TransformSchemaNode logic in OpenApiSchemaService to handle FileContentResult with the binary format

Fixes #63172

[Copilot]

Pull Request Overview

This PR fixes OpenAPI schema generation for FileContentResult to properly represent it as binary data instead of a JSON object reference. The change ensures that when FileContentResult appears in API responses, the generated OpenAPI schema correctly indicates it's a binary file format.

• Modified schema transformation logic to include FileContentResult alongside other binary types like IFormFile, Stream, and PipeReader
• Updated the schema ID assignment to distinguish FileContentResult from other binary types

[dotnet-policy-service]

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

[captainsafia]

This PR has build failures (missing usings) that need to be addressed.

[martincostello]

This has changed the meaning of the code. It was returning the same value for all types before, now they're all different. I doubt this is correct, but even if it is it will cause tests to fail so will need further changes.

I suggest you follow the [guide to build and test the code](https://github.com/dotnet/aspnetcore/blob/main/docs/BuildFromSource.md aspnetcore/docs/BuildFromSource.md) and change the code using an IDE and run the relevant tests so you can check none get broken, and also so you can add a test for this change.