Use visualization dropdown solely for examples

[guerler]

Follow-up on suggestions made in #20046. Revises the example datasets dropdown in the visualizations creation form:

Additionally, this PR adds more tests.

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. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

License

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

[davelopez]

Thank you for the extra tests and type improvements!