Collaborative editor crashes when saving job without credential

[stuartc]

User story

When I create a job in the collaborative editor and don't select a credential, the workflow fails to save with a validation error.

Details

The job form sends undefined for empty credential fields. The backend expects nil or a valid credential ID, so the save fails.

This blocks users from creating jobs that don't need credentials (like simple data transformations with the common adaptor).

The job form also lacks test coverage in both unit and E2E tests. The LiveView workflow editor has tests in workflow-steps-creation.spec.ts, but the React collaborative editor doesn't.

Implementation notes

1. Add unit tests for the JobInspector component
2. Add E2E tests that cover the credential selection scenarios
3. Fix the bug so jobs can be saved without credentials

Release notes

Jobs in the collaborative editor can now be saved without selecting a credential.

User acceptance criteria

• Jobs can be saved without selecting a credential
• Jobs can be saved with a project credential
• Jobs can be saved with a keychain credential
• Credentials can be cleared after being selected
• Unit tests cover the JobInspector form behavior
• E2E tests verify the credential scenarios work end-to-end