Add ResponseDecodeError for richer client decode failure debugging

[cubahno]

Summary

• Adds ResponseDecodeError struct to pkg/runtime with structured fields (StatusCode, ContentType, ContentLength, TargetType, Body) for programmatic inspection of decode failures
• Updates client.tmpl to return &ResponseDecodeError{...} instead of generic fmt.Errorf("error decoding response: %w", err) at both error-response and success-response unmarshal paths
• Body is intentionally excluded from .Error() to prevent PII leakage into logs, but accessible via errors.As for developers who need it

Motivation

The previous error message - error decoding response: <unmarshal error> - was hard to debug. It lacked context about what HTTP status was received, what content type the server returned, how large the body was, or what Go type was being targeted. A common suggestion was to
include the body in the error string, but that risks leaking sensitive data into logs and error tracking systems.

ResponseDecodeError gives everything needed:

• The .Error() string includes metadata for log-friendly debugging
• errors.As gives programmatic access to all fields including the raw body, when the developer explicitly opts in