• closes BUG: OverflowError when fillna on DataFrame with a pd.Timestamp #61208
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions. (Does not apply)
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

Fix for fillna with Out-of-Bounds Datetime Values

Issue: Using fillna on a datetime64[ns] column with an out-of-bounds timestamp (e.g., '0001-01-01') raised an AssertionError instead of the expected OutOfBoundsDatetime.

Fix: Modified the putmask method in pandas/core/internals/blocks.py to catch and re-raise OutOfBoundsDatetime directly, preventing the AssertionError.

Test Added:

• Created test_fillna_out_of_bounds_datetime in pandas/tests/frame/methods/test_fillna.py.
• The test:
  • Sets up a DataFrame with a datetime64[ns] column containing NaT.
  • Attempts to fill NaT with '0001-01-01'.
  • Expects OutOfBoundsDatetime.