Add the duration field to the file upload responses

[gpunto]

🔗 Issue Links

Surfaced by GetStream/stream-chat-android#6632, which switches Android's uploads to the OpenAPI-generated model.

• The four file/image upload routes returned only file. In the backend OpenAPI specs, duration is the only required property of FileUploadResponse/ImageUploadResponse — file and thumb_url are optional — so every client generated from them has duration mandatory and fails to decode the mock response (on Android: Required value 'duration' missing at $).
• Added a shared upload_response helper and used it in all four routes. thumb_url stays omitted, as the real backend omits it when no thumbnail was generated.
• Those routes built their body with .to_s (Ruby hash-inspect, not JSON) and now use .to_json. The ~16 other .to_s bodies in endpoints.rb are a follow-up.
• Verified by POSTing all four routes against a local server: valid JSON with file and duration.

🧪 Testing Notes

Please verify the E2E test runs:

• iOS
  • run 31574861012
• Android
  • run 31574863162 — green on all 7 API levels, AttachmentsTests included
• Flutter
  • run 31574865177 — fails, but not because of this branch: last night's nightly on mock main fails identically (message_list_test/reactions_test, same counts, same RawTooltipState is a SingleTickerProviderStateMixin but multiple tickers were created assertion). No attachment test involved and no upload decode error in the logs.

[testableapple]

LGTM