#133 fix: surface 422 validation error messages in SendDraft and UpdateDraft

The generated client parses 422 responses into JSON422, but the service wrappers (UpdateDraft and SendDraft) discarded this and fell back to CheckResponse which returns a generic validation error string. Parse JSON422 and return specific messages from the response body instead.

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Summary by cubic

UpdateDraft and SendDraft now return the specific validation error messages from HEY API 422 responses instead of a generic "validation error" string. The error keeps the HTTP status and X-Request-Id, and falls back to the generic message when the response body carries no reasons.

• Adds conformance tests for both operations answering 422 with a reasons list.