Don't calculate dataset hash for datasets in non-OK state

[nsoranzo]

Also:

• Check again that the dataset hasn't been purged right before hash calculation starts.

Address comments in
https://github.com/galaxyproject/galaxy/pull/19181/files#r1853605980

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.

[mvdbeek]

Thank you!