Implements Go SDK generator with two-level fallback:
1. Native oapi-codegen if installed (generates types and client)
2. Docker openapitools/openapi-generator-cli as fallback
Native generation also creates a minimal go.mod file.
Includes tests following _Good convention for Available and Generate.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
