feat: typed callbacks and errors - #17

Summary

Orchestrator v2.2.0 reports a failed callback's error as {code, message}. This PR gives callbacks a typed surface end to end.

CloudEvent<T> is generic and knows nothing about its payload. CloudEvent::decode($payload, $decode) hands the event type and raw data to a callable and stores whatever comes back; fromArray() is decode() with the identity, so existing raw-array callers are unchanged.

One Callback per event type, dispatched by CallbackEvent::decode(array $data):

Callback\Error (ErrorCode $code, string $message) with Enum\ErrorCode covering every artifact, exit, and deployment-response code. Strict: a non-object error, unknown code, or missing message throws ClientException. No tolerance for the pre-2.2 string shape — pin the SDK to the orchestrator it talks to.

$event = CloudEvent::decode(
    \json_decode($rawBody, true),
    fn (string $type, array $data) => CallbackEvent::from($type)->decode($data),
);

if ($event->data instanceof JobArtifact && $event->data->error !== null) {
    $log->error("{$event->data->artifactId}: {$event->data->error->message}");
}

Also adds Data::bool for the response callback's truncation flags.

Test plan

• composer test — each event type decodes to its callback; artifact/exit/response error cases; unknown event type and missing fields rejected; Error object, null-on-success, and malformed shapes
• composer analyze, composer format:check, composer refactor:check

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