Specify a Content-Type header on stream type requests when th epayload type is known

[Numpsy]

refs #291

I believe that the content-type header needs to be set on the Content itself rather than in the general request headers?

This is done already for text and (I think) form type posts, but not for streams.
There is code at

SwaggerProvider/src/SwaggerProvider.Runtime/RuntimeHelpers.fs

Line 221 in d9e71a0

if not <| msg.Headers.TryAddWithoutValidation(name, value) then

which tries to set the Content-Type at the request level but the values don't seem to actually be sent in the request from my testing.
Maybe fillHeaders could be changed to handle the content-type headers differently, but I think this way is more consistent with how toStringContent and toTextContent work.

[Copilot]

Pull request overview

This pull request fixes an issue where Content-Type headers were not being properly set on stream-based HTTP requests. The fix ensures that when a payload type is known for stream requests, the Content-Type header is set directly on the StreamContent object rather than only on the request headers.

Changes:

• Modified toStreamContent function to accept a content type parameter and set it on the StreamContent's headers
• Updated the call site in OperationCompiler to pass the payload MIME type to toStreamContent
• Enhanced test server endpoint to validate that Content-Type headers are being sent correctly

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/SwaggerProvider.Runtime/RuntimeHelpers.fs	Added contentType parameter to toStreamContent and logic to set Content-Type header on StreamContent when provided
src/SwaggerProvider.DesignTime/v3/OperationCompiler.fs	Updated toStreamContent call to pass payloadMime as the content type parameter
tests/Swashbuckle.WebApi.Server/Controllers/FileController.fs	Modified test endpoint to validate Content-Type header is "application/octet-stream" and return appropriate error status when missing

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