Validate ds range early; clear error for out-of-bounds datetime64[ns]

[NikhithReddyScripts]

Summary

Prophet crashed when ds contains dates outside pandas datetime64[ns] range. Example: 3969-12-02.

Root cause

preprocess() casts ds via pd.to_datetime(...), which forces ns precision and raises OutOfBoundsDatetime deep in pandas.

Fix

Add _validate_ds_ns_range(df["ds"]) at the start of preprocess(). It parses to plain date without pandas and raises a clear ValueError with the supported range (1677-09-21 to 2262-04-11) and the first offending value.

Tests

• test_in_range_ok verifies normal fit with valid dates.
• test_out_of_range_raises asserts ValueError and message substring datetime64[ns].

Behavior

No changes for valid inputs. Out-of-range data now gets an immediate, actionable error instead of a pandas traceback.

Fixes #2689.

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