has_size assertion: implement size (as synonym for value)

[bernt-matthias]

fixes https://github.com/galaxyproject/galaxy/issues/17189

How to test the changes?

(Select all options that apply)

• [x] 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

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