Code cleanups/improvements

Problem/Motivation

Currently there are some code segments that need some cleanup and improvements. This ticket is used to test all these changes with the CI

Steps to reproduce

n/a

Proposed resolution

Clean up / improve code

Remaining tasks

• Avoid multiple calls to $field->getViewMode() in view mode switch service
• Do not override passed in $view_mode variable when checking for subsequent view modes in view mode switch helper service
• Avoid multiple calls to $field->getName() in view mode switch service
• Improve formatter test by removing duplicate method calls and use a dedicated field for each formatter
• Replace usages of deprecated assertEqual() in tests
• Split up view mode helper service to have a dedicated test for entity view mode delete confirm form helpers
• Set correct return type hints for all functions/methods

User interface changes

n/a

API changes

n/a

Data model changes

n/a