Skip convenience method generation for multipart/mixed requests

[Copilot]

The C# emitter does not support multipart/mixed requests, yet since the addition of multipart/form-data support it would emit an incorrect convenience method for them. This disables convenience method generation for multipart content types other than multipart/form-data and surfaces a diagnostic instead.

The detection lives in the emitter so the generator simply honors the resulting generateConvenienceMethod flag.

Changes

• operation-converter.ts: In fromSdkServiceMethodOperation, when an operation uses a multipart/* content type other than multipart/form-data, generateConvenienceMethod is set to false (protocol methods still emitted) and a warning diagnostic is reported. A small isUnsupportedMultipart helper inspects the request media types.
• lib.ts: Added the unsupported-multipart-convenience-method warning diagnostic.
• Tests: Added emitter unit tests in operation-converter.test.ts — multipart/mixed turns off convenience method generation and emits the warning, while multipart/form-data keeps convenience methods with no diagnostic.

const requestMediaTypes = getRequestMediaTypes(method.operation);
if (generateConvenience && isUnsupportedMultipart(requestMediaTypes)) {
  diagnostics.add(
    createDiagnostic({
      code: "unsupported-multipart-convenience-method",
      format: { methodCrossLanguageDefinitionId: method.crossLanguageDefinitionId },
      target: method.__raw ?? NoTarget,
    }),
  );
  generateConvenience = false;
}

No existing test specs use multipart/mixed, so generated library output is unchanged.

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@typespec/http-client-csharp@10967

commit: a434666

[jorgerangel-msft]

@copilot ensure we add proper unit tests for our changes

[jorgerangel-msft]

@copilot ensure we are regenerating all test libraries

[Copilot]

@copilot ensure we add proper unit tests for our changes

Added emitter unit tests in operation-converter.test.ts: one verifies multipart/mixed turns off generateConvenienceMethod and emits the unsupported-multipart-convenience-method warning, and another verifies multipart/form-data keeps convenience methods with no diagnostic.

[Copilot]

@copilot ensure we are regenerating all test libraries

Ran eng/scripts/Generate.ps1 to regenerate all test libraries. No generated output changed since no test specs use multipart/mixed.