fix(KONFLUX-5886): allow unselecting default context

[CryptoRodeo]

The user should be able to unselect the default context during the creation of an integration test.

Other changes:

• Contexts are now required (at least one)

[CryptoRodeo]

Demo of the following:

• default context can now be unselected (main bug fixed)
• contexts are now required by the form and modal (at least 1 must be selected).

konflux-5886.mp4

[CryptoRodeo]

It appears that the code coverage step fails:

error - 2024-12-18 21:48:17,362 -- Report creating failed: {"message":"Token required because branch is protected"}
Error: Codecov:
                        Failed to properly create report: The process '/home/runner/work/_actions/codecov/codecov-action/v4/dist/codecov' failed with exit code 1

[testcara]

I met the same token problem for my ci jobs but after 2 days later, retry fixed the problem. It seems the ci is not very stable for this part.

[testcara]

LGTM

[JoaoPedroPP]

LGTM