BUG: Fix assert_frame_equal with check_dtype=False for pd.NA dtype differences (GH#61473)
- [x] closes #61473
- [x] tests added / passed
- [x] Ensure all linting tests pass
- [x] whatsnew entry

## Problem
When comparing two DataFrames containing `pd.NA` values with `check_dtype=False`, `assert_frame_equal` fails when the DataFrames only differ in dtype (object vs Int32). This happens because `pd.NA` and `np.nan` are treated as different values even though they represent the same missing value.

## Solution
Modified `assert_frame_equal` in `pandas/_testing/asserters.py` to normalize `pd.NA` and `np.nan` values when `check_dtype=False` is specified. This ensures that DataFrames with equivalent missing values but different dtypes can be compared successfully.

## Changes Made
- Added normalization logic in `assert_frame_equal` function to handle `pd.NA` and `np.nan` equivalence when `check_dtype=False`
- Added comprehensive unit test in `pandas/tests/util/test_assert_frame_equal.py` to verify the fix

## Testing
- Added unit test `test_assert_frame_equal_pd_na_dtype_difference` that reproduces the original issue and verifies the fix
- Test passes successfully with the implemented solution
Hello,

It seems that the "Downstream Compat" CI job is failing with an `ImportError` related to `statsmodels` and `scipy`. This appears to be an issue with the CI environment's dependencies (e.g., version mismatch between `statsmodels` and `scipy`), and not directly related to the changes introduced in this PR.

The error message is:
```
_______________________________ test_statsmodels _______________________________
E       pytest.PytestDeprecationWarning: 
E       Module 'statsmodels.formula.api' was found, but when imported by pytest it raised:
E           ImportError("cannot import name '_lazywhere' from 'scipy._lib._util' (/home/runner/micromamba/envs/test/lib/python3.11/site-packages/scipy/_lib/_util.py)")
```

I believe this is an infrastructure/dependency issue on the CI side, rather than a bug in my proposed changes for GH#61473.

Could you please confirm this and let me know if there's anything I can do to help resolve this CI issue, or if this is something the core `pandas` team needs to address? 