[swift6] Send raw Data multipart parts as files

[spigo]

Part of #21444; complements #22456.

A raw Data multipart value was appended with only a name, so servers expecting a file upload (a part with a filename, e.g. $_FILES in PHP or MultipartFile in Spring) didn't receive a file. Data parts now get filename (the parameter name) and Content-Type: application/octet-stream:

• alamofire: mpForm.append(data, withName: k, fileName: k, mimeType: "application/octet-stream")
• urlsession: configureDataUploadRequest takes optional fileName/mimeType, passed only for Data values

String, number and UUID values stay plain form fields. URL values keep their existing file handling.

Verified with the generated default (urlsession) sample by capturing the request body through an OpenAPIInterceptor: the body for ["file": Data(...), "note": "hello"] now contains name="file"; filename="file" with Content-Type: application/octet-stream, and name="note" unchanged. The alamofire variant has been running in a downstream client against a PHP backend. The default and alamofireLibrary samples build.

Note: until #22456 lands, generated API methods still base64-encode Data form params before they reach the builder (via asParameter). This change takes effect for every raw Data value that reaches the builder, and for all of them once #22456 is merged.

PR checklist

• Read the contribution guidelines.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/swift6*.yaml || exit
  

  Commit all changed files.
• @mention the technical committee: @jgavris @ehyche @Edubits @jaz-ah @4brunu @dydus0x14

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Summary by cubic

Raw Data multipart values were sent as plain form fields, so servers expecting a file upload (a part with a filename) didn't receive one. Data parts now include filename (the parameter name) and Content-Type: application/octet-stream in both the alamofire and urlsession libraries, while String, number, and UUID values stay plain form fields.

^{Written for commit ef8ec9b. Summary will update on new commits.}

[cubic-dev-ai]

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