There is an art to writing tests especially getting absraction levels right. For example do you integration test hitting the password field with 1000 cases or do that as a unit test, and does doing it as a unit test sufficiently cover this.

AI could do all this thinking in the future but not yet I believe!

Let alone the codebase is likely a mess of bad practice already (never seen one that isn't! That is life) so often part of the job is leaving the campground a bit better than how you found it.

LLMs can help now on last mile stuff. Fill in this one test. Generate data for 100 test cases. Etc.