Add missing types to visualization model

[guerler]

Add missing typing to the visualization model and tests to ensure all required attributes are serialized.

How to test the changes?

(Select all options that apply)

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. Navigate to a visualization
  2. Notice the missing help content

License

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.