These are good questions! Direct responses:

1) We aren't validating title length - we are validating that the server returns the correct error when the title length is invalid.

2) What if one of the tools is modified and the validation breaks, but doesn't throw any errors?  The only indication would be that the API doesn't function according to the design spec - thus why we must test it here.

3) Again, this is testing the API, not the internal tools.  Will the API act properly in an error case?  That's what we're testing.

4) They are supposed to, but the purpose of these tests is to confirm that assumption.