Mattpollock fix/r client error handling

[wing328]

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Summary by cubic

Fixed R client error handling to return consistent ApiResponse objects with content, response, and status_code. Corrected empty-response checks to handle vector values and updated generated R samples.

• Bug Fixes
  • Construct ApiResponse with named fields and include status_code in non-2xx branches.
  • Standardize messages: 3xx uses “Server returned …”, 4xx uses “API client error”, 5xx falls back to “API server error” only when response is NULL or all empty.
  • Use all(local_var_resp$response == "") to avoid logical vector issues; regenerate echo_api and petstore R samples.

^{Written for commit c81729d45ada6558e58696f830f2ea50c3a92da9. Summary will update on new commits.}