Fix path serialization in form request

[hendraet]

Description

Form fields were not able to properly serialize paths when get_step_form was called. The CustomEncoder was updated to also include paths. I also added some tests that should fail if future code adds data types to fields that cannot natively be serialized for JSON.

Testing

Create a workflow that contains a step, which uses a file upload (e.g., PTM Visualization: Bar Plot). After uploading a file, it should be possible to navigate away and back to the step again, which led to an error before.

PR checklist

Development

• If necessary, I have updated the documentation (README, docstrings, etc.)
• If necessary, I have created / updated tests.

Mergeability

• dev-branch has been merged into local branch to resolve conflicts
• The tests and linter have passed AFTER local merge
• The backend code has been formatted with black
• The frontend code has been formatted with pnpm format and checked with pnpm lint

Code review

• I have self-reviewed my code.
• At least one other developer reviewed and approved the changes

[github-actions]

Coverage report

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
backend/protzilla
form.py
Project Total

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[Elena-kal]

Code looks fine and it works. Nice :))