#94 Honor per-operation retry policies in the generated client

What changed

The generated Go client now reads each operation's x-hey-retry contract:

• maxAttempts sets the operation default attempt budget
• retryOn sets the operation-specific HTTP status list
• explicit WithRetryConfig still overrides the attempt count, while retryable statuses remain contract-defined
• the credential-refresh resend and Hooks.OnRetry behavior added by #137 and #139 remain intact

The patch is limited to the Go client template, its regenerated output, and one Go-only conformance case plus the runner support needed to discover it. The case uses DeleteBoxDesignation: one call stops immediately on undeclared 500, then a second call retries declared 503 once and stops at the operation's two-attempt limit before a 200 sentinel.

Rust retry behavior and the higher-level hey.Client default retry override are intentionally unchanged.

Validation

• GOWORK=off mise exec -- make check
• Go conformance: 188/188
• Rust conformance: 187/187
• generated output reproduces byte-for-byte