fix(web-api): align files.uploadV2 return type

[kenkoooo]

Summary

• align the public files.uploadV2 method to advertise the same return payload as the helper filesUploadV2
• update the docs and tsd coverage so callers can rely on the files array in the response

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @kenkoooo to sign the Salesforce Inc. Contributor License Agreement.