Fix HostedFileContent with image MIME type sent as input_file instead of input_image

[Copilot]

HostedFileContent with image media types (e.g. image/png) was unconditionally mapped to CreateInputFilePart, causing OpenAI to reject it with: "Expected a file with an application/pdf MIME type, but got unsupported MIME type 'image/png'".

The tool output path already handled this correctly via HasTopLevelMediaType("image") — the user message path did not.

Changes

• OpenAIResponsesChatClient.cs: Added a HostedFileContent case guarded by HasTopLevelMediaType("image") before the generic fallback, routing image files through CreateInputImagePart (with detail level support)
• OpenAIResponseClientTests.cs: Extended UserMessageWithVariousContentTypes_ConvertsCorrectly to cover hosted image files with and without detail level

// Before: always input_file
case HostedFileContent fileContent:
    (parts ??= []).Add(ResponseContentPart.CreateInputFilePart(fileContent.FileId));
    break;

// After: image-aware routing
case HostedFileContent fileContent when fileContent.HasTopLevelMediaType("image"):
    (parts ??= []).Add(ResponseContentPart.CreateInputImagePart(fileContent.FileId, GetImageDetail(item)));
    break;

case HostedFileContent fileContent:
    (parts ??= []).Add(ResponseContentPart.CreateInputFilePart(fileContent.FileId));
    break;

[Copilot]

Pull request overview

Fixes OpenAI Responses “user message” content conversion so HostedFileContent with an image MIME type (e.g., image/png, image/jpeg) is sent as input_image (optionally with detail) instead of incorrectly being sent as input_file (which can be rejected by OpenAI).

Changes:

• Route HostedFileContent with top-level media type image/* through ResponseContentPart.CreateInputImagePart(...) in OpenAIResponsesChatClient.
• Extend UserMessageWithVariousContentTypes_ConvertsCorrectly to validate hosted image files are serialized as input_image (with and without detail).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIResponsesChatClient.cs	Adds an image-aware HostedFileContent switch case so hosted image files map to input_image rather than input_file.
test/Libraries/Microsoft.Extensions.AI.OpenAI.Tests/OpenAIResponseClientTests.cs	Updates the verbatim request JSON + content list to assert hosted image files are serialized as input_image (including detail).