fix: let isset() and ?? see the JSON fields of a Payload

[oniric85]

Payload exposes the fields of a JSON body as properties through __get, but
it never declared __isset. PHP consults __isset, not __get, for isset(),
empty() and ??, so every one of those checks on a Payload reported the
field as missing while a plain read returned it. Client::api() tests
$result->error ?? false and so returned a JetStream error reply as a
success instead of throwing, and a caller that inspects a PubAck the same
way takes a rejected publish for an acknowledged one.

__isset now answers true when the body has the field and it is not null,
the rule isset() applies to a real property. __get is unchanged.
Client::api() throws on an error reply as it was written to do; the
functional suite is unaffected by that.

Co-Authored-By: Claude Fable 5.1 noreply@anthropic.com

🤖 Generated with Claude Code

[oniric85]

Hi Mattias, I opened this small PR.

Payload has __get but no __isset, so isset(), empty() and ?? on its JSON fields always say "not set". Client::api() checks $result->error ?? false, so a JetStream error reply comes back as a success, and JetStreamAck::fromRawAck() in php-event-client has the same problem: a rejected publish looks acknowledged. We hit it on EC-4594 ticket, where ICF signed events got lost with "handled successfully" in the logs.

The PR adds __isset with the same rule PHP applies to real properties (set and not null), plus two tests. The functional suite is unchanged.

Could you review it when you have a moment? And, would a tag with just this fix be possible, or does it have to wait for other work? We need to bump php-event-client after it and then eConsent.