Test suite source validation

[petertrr]

After test suite source is fetched, we should store and display validations, including among others:

• Whether source contains suites with plugins that have zero tests
• Whether source contains suites with wildcard mode (not handled well by save-cloud currently?)

This includes calculating these validations, storing them in the storage and support them on the frontend

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Tasks, as discussed with @nulls:

• Front-end
  • Implement the useEventStream() (SSE, HTTP GET only) and useNewlineDelimitedJson() React hooks #1810
  • Display the current fetch status ("Fetching...", "Completed", "Errors encountered", "Time-out", "Preprocessor unavailable") when (re-)fetching
  • Display the last fetch status ("Completed", "Errors encountered") when viewing tests (the Tests tab within an organization), using data from the tests_source_version and tests_source_snapshot tables.
• Back-end
  • TBD
• Preprocessor
  • Wrap blocking operations into blockingToMono {} #1778
  • Implement test suite validators on the preprocessor side #1955
  • Get rid of type aliases #1956
  • Refactor TestSuitesPreprocessorController#doFetchTests() and the underlying calls
    • The snapshot should be always saved (TestPreprocessorToBackendBridge#saveTestsSuiteSourceSnapshot()), even in case of validation errors. This is necessary, so that a malformed snapshot is never re-fetched again.
    • The tests_source_snapshot table, along with the corresponding DTO, should be enriched with isSuccessful and errorMessage fields
    • TestDiscoveringService#detectAndSaveAllTestSuitesAndTests() should perform the following operations, in order:
      1. detect test suites and tests
      2. validate test suites and tests
      3. update the saved snapshot with status and error messages, if any
      4. save test suites and tests, if the validation succeeded