Add return type of kerberos.py and Create test of test_perform_krb181_workaround

[makinzm]

What I did

1. Type annotation correction:
   • In the Airflow codebase, a return type was incorrectly annotated as None when it actually returns an int.
   • While Python allows for flexible typing, accurate type annotations improve code clarity and catch potential errors early.
2. Test coverage improvement:
   • Some existing function tests were not comprehensive.
   • Added more thorough unit tests to cover all conditional branches of the affected function(s).

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[boring-cyborg]

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