fix codegen client compatibility

[fergusean]

Summary

• correct RequestResult so async request wrappers return one promise of the possible result shapes
• add regression coverage ensuring optional-upload requests without a file use JSON
• update error-handling examples for responses that may be unavailable after network failures
• bump the package to 2.9.1

Root cause

The legacy client generator selected multipart encoding for endpoints that also accepted JSON, causing nested request fields to arrive as strings unless a file activated the _payload upload adapter. The current generator selects JSON when the optional upload is omitted. During the consumer upgrade, the v2.9.0 RequestResult conditional type also exposed an incompatible union-of-promises shape for normal async wrappers.

Validation

• yarn format
• yarn lint
• yarn test:typecheck
• yarn build