Improve validation of the test data for fragment inputs

[mgoworko]

Describe your changes

There was an issue with input validation during fragment testing. Fragment input parameters can be configured with additional validation rules, such as being marked as required or restricted to a predefined set of values.

In the test mechanism, test data samples are generated by the mocked source and become available only after the source is invoked. However, for fragments, validation is executed before the source is invoked. As a result, validation of fragment input parameters always failed, when the parameters were marked as required.

Solution:

• always validate the test input before starting test execution. This was already partially implemented by calling the validation endpoint on the frontend, but validation is now also executed immediately before the test is performed.
• disable validation for mocked fragment input sources during test execution

Checklist before merge

• Related issue ID is placed at the beginning of PR title in [brackets] (can be GH issue or Nu Jira issue)
• Code is cleaned from temporary changes and commented out lines
• Parts of the code that are not easy to understand are documented in the code
• Changes are covered by automated tests
• Showcase in dev-application.conf added to demonstrate the feature
• Documentation added or updated
• Added entry in Changelog.md describing the change from the perspective of a public distribution user
• Added MigrationGuide.md entry in the appropriate subcategory if introducing a breaking change
• Verify that PR will be squashed during merge

[piotrp]

Please add a changelog entry