Fix incorrect return type annotations in darts.utils.statistics

[AlejandroCoronadoN]

Three functions in darts/utils/statistics.py have return type annotations that do not match what they actually return:

• stationarity_test_kpss is annotated -> set but returns a tuple (it wraps statsmodels.tsa.stattools.kpss).
• stationarity_test_adf is annotated -> set but returns a tuple (it wraps adfuller).
• granger_causality_tests is annotated -> None but returns a dict (it wraps grangercausalitytests); its own docstring already says it returns a Dict.

The docstrings and the existing tests already treat these as tuples/dict (for example stationarity_test_kpss(series)[1]), so the annotations are just wrong and mislead type checkers and IDEs. This fixes the annotations to match the real return values and adds a small regression test.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.98%. Comparing base (2be8e43) to head (6c1b008).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3185      +/-   ##
==========================================
- Coverage   97.04%   96.98%   -0.07%     
==========================================
  Files         165      165              
  Lines       17976    17976              
==========================================
- Hits        17445    17434      -11     
- Misses        531      542      +11     

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