Upgrade MLFlow integration tests to 2.9.2

[jgiannuzzi]

This PR upgrades our MLFlow integration tests to 2.9.2, enables a few additional tests (artifacts and path validation), disables the SQLite memory backend, and makes a few changes to our code for increased compatibility with MLFlow:

• the file:// prefix gets added to the default artifact root when a local path is provided
• a distinction is made between generic SQL errors and a run not being found when trying to delete a run tag
• the error message returned for an invalid path has been changed to "Invalid path" (and the validation logic has been DRYed up)
• the error message when an inexistent run is provided has been changed to "Run 'id' not found"
• the error message when an inexistent tag is provided to DeleteRunTag has been changed to "No tag with name: key"
• the validator for LogBatch is more exhaustive

Our own tests have also been updated to check for these changes.