Remove usage of static vars or properties

[come-nc]

Summary

Static properties, and even worse static vars in methods are hard to reset from outside, and become a major problem when wanting to make use of the worker mode of FrankenPHP or any similar project.

So this PR removes such declarations and adds a psalm custom plugin to forbid their use.

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)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI

[come-nc]

Error: lib/private/TextProcessing/Manager.php:140:13: InvalidArrayOffset: Cannot access value on variable OC\TextProcessing\Manager::COMPATIBLE_TASK_TYPES using a class-string<OCP\TextProcessing\ITaskType> offset, expecting 'OCP\\TextProcessing\\FreePromptTaskType'|'OCP\\TextProcessing\\HeadlineTaskType'|'OCP\\TextProcessing\\SummaryTaskType'|'OCP\\TextProcessing\\TopicsTaskType' (see https://psalm.dev/115)
Error: lib/private/TextProcessing/Manager.php:228:10: InvalidArrayOffset: Cannot access value on variable OC\TextProcessing\Manager::COMPATIBLE_TASK_TYPES using a class-string<OCP\TextProcessing\ITaskType> offset, expecting 'OCP\\TextProcessing\\FreePromptTaskType'|'OCP\\TextProcessing\\HeadlineTaskType'|'OCP\\TextProcessing\\SummaryTaskType'|'OCP\\TextProcessing\\TopicsTaskType' (see https://psalm.dev/115)
Error: lib/private/TextProcessing/Manager.php:229:67: InvalidArrayOffset: Cannot access value on variable OC\TextProcessing\Manager::COMPATIBLE_TASK_TYPES using a class-string<OCP\TextProcessing\ITaskType> offset, expecting 'OCP\\TextProcessing\\FreePromptTaskType'|'OCP\\TextProcessing\\HeadlineTaskType'|'OCP\\TextProcessing\\SummaryTaskType'|'OCP\\TextProcessing\\TopicsTaskType' (see https://psalm.dev/115)

No idea how to explain to psalm that this is fine 🤷

[come-nc]

This fails in tests because the listener gets the singleton from container and tries to re-register the backend.
It can only happen if code creates BackendService objects.

Not sure how to best fix it.
Ideally we should move to a typed event with a pointer to the backend service. But that cannot be done right away.

Maybe we should get the singleton from container and check eventSent on it?