ErrorPayload.retryable omitted from wire when null (§12)

[nficano]

Category: spec-conformance Severity: major
Location: src/Errors/ErrorPayload.php:79-98
Spec: ARCP v1.1 §12

What

§12: 'Error payloads carry a retryable boolean. LEASE_EXPIRED and BUDGET_EXHAUSTED MUST be returned with retryable: false.' Because the constructor defaults retryable to null and toArray suppresses null, any caller building new ErrorPayload('LEASE_EXPIRED', '...') emits a wire envelope without the required boolean.

Evidence

public function toArray(): array
{
    $out = [
        'code' => $this->code,
        'message' => $this->message,
    ];
    if ($this->retryable !== null) {
        $out['retryable'] = $this->retryable;
    }
    ...
}

Proposed fix

When retryable is null at construction, derive it from ErrorCode::tryFrom($code)?->defaultRetryable() and always emit the field. Hard-code retryable=false for LEASE_EXPIRED and BUDGET_EXHAUSTED, retryable=true for INTERNAL_ERROR.

Acceptance criteria

• Every encoded error payload contains a retryable boolean; LEASE_EXPIRED and BUDGET_EXHAUSTED carry retryable:false; INTERNAL_ERROR carries retryable:true.