refactor(RichObjectStrings): Only log error if key or value is not string in validator

[provokateurin]

Fixes #51427
Fixes nextcloud/activity#1875

Summary

#47662 was a bit too hard and has caused quite a few issues because there are so many places that are wrong.
While it makes developers more aware, it also breaks a lot of users which is not good. Therefore I'm downgrading it to an error message for now, so developers hopefully still notice it.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[provokateurin]

/backport to stable31

[susnux]

Should add the expected logger call here instead

[juliusknorr]

This is causing some log spam on our instance:

Some rare cases happened before, but not in that amount.

Edit: Seems the new log catches more code paths where is was probably catched before

root@cloud1:~# grep "Object for placeholder" /var/www/data/nextcloud.log* | grep 2025-04-08T19 | cut -d ':' -f 2- | jq .url | sort -h | uniq -c
      2 "--"
   9077 "/ocs/v2.php/apps/notifications/api/v2/notifications"
    854 "/ocs/v2.php/apps/notifications/api/v2/notifications?format=json"
root@cloud1:~# grep "Object for placeholder" /var/www/data/nextcloud.log* | grep 2025-04-08T0 | cut -d ':' -f 2- | jq .url | sort -h | uniq -c
     39 "/ocs/v2.php/apps/dashboard/api/v2/widget-items?widgets%5B%5D=activity"

[nickvergessen]

Yeah it was muted and caught in activity and notifications:

server/lib/private/Activity/Event.php

Lines 429 to 433 in 3c698c6

	try {
	$this->richValidator->validate($this->getRichSubject(), $this->getRichSubjectParameters());
	} catch (InvalidObjectExeption $e) {
	return false;
	}

server/lib/private/Notification/Notification.php

Lines 442 to 446 in 5193579

	try {
	$this->richValidator->validate($this->getRichSubject(), $this->getRichSubjectParameters());
	} catch (InvalidObjectExeption $e) {
	return false;
	}