Klantcontact notifications fail

[JanBrek]

Product versie / Product version

2.18.0

Omschrijf het probleem / Describe the bug

Original feature: #598

Klantcontact notifications fail because OpenKlant sends values as booleans but OpenNotificaties expect strings.

Stappen om te reproduceren / Steps to reproduce

1. Use the register kanalen on CLI to let OpenKlant register itself with OpenNotificaties
2. Make a Klantcontact via the API
3. The worker will try to send (Taken from our T env, but URLs obfuscated) '{"kanaal": "klantcontacten", "hoofdObject": "https://{URL}/klantinteracties/api/v1/klantcontacten/xxxx", "resource": "klantcontact", "resourceUrl": "https://{URL}/klantinteracties/api/v1/klantcontacten/xxx", "actie": "create", "aanmaakdatum": "2026-08-31T16:07:05.805746Z", "kenmerken": {"hoofdOnderwerpType": "", "indicatieContactGelukt": true, "verdereActieOndernomen": false}}'
4. Which returns an error from openNotificaties: '{"kenmerken":{"indicatieContactGelukt":["Not a valid string."],"verdereActieOndernomen":["Not a valid string."]}}'

(I can deliver the full logs but not via a public ticket :) )

Verwacht gedrag / Expected behavior

2 options

1. OpenKlant sends strings
2. OpenNotification accepts booleans

[alextreme]

@Floris272 zie eerdere communicatie