fix: precise_diff inconsistent with pendulum.DateTime subclasses (#906)

[r266-tech]

Problem

When pendulum.interval() is called with native datetime.datetime objects, the resulting interval produces incorrect values for in_words(), hours, minutes, etc. This is because Interval.__init__ converts native datetimes to pendulum.DateTime, but the underlying Rust precise_diff function handles them inconsistently.

Example:

from datetime import datetime
import pendulum

start = datetime(2025, 7, 25, 19, 26, 34)
end = datetime(2025, 7, 29, 19, 26, 34)
interval = pendulum.interval(start, end)

# Before fix:
interval.in_words()   # "3 days 4 hours 33 minutes" (WRONG)
interval.hours        # 4 (WRONG)

# After fix:
interval.in_words()   # "4 days" (CORRECT)
interval.hours        # 0 (CORRECT)

Root Cause

In rust/src/python/helpers.rs, the precise_diff function uses inconsistent type checks:

• Line 174: is_datetime: PyDateTime::is_type_of(dt1) — returns True for pendulum.DateTime (subclass)
• Line 187: is_datetime: PyDateTime::is_exact_type_of(dt2) — returns False for pendulum.DateTime

When dt2.is_datetime is False, its hour/minute/second components are left at 0, producing an incorrect diff.

Fix

Changed line 187 from is_exact_type_of to is_type_of to match line 174.

Testing

• Added regression test test_native_datetime_interval_consistency that verifies native and pendulum datetime inputs produce identical results
• All 42 existing interval tests pass

Fixes #906