Fix arithmetic with standard timedelta operands

[Eric3-jp]

Fixes #382.

Duration accepts datetime.timedelta operands in division and remainder operations, but calls the operand's private _to_microseconds() method. Standard-library timedeltas do not have that method, so /, //, %, and divmod() raise AttributeError. The same problem affects Interval through its duration arithmetic.

Convert standard-library timedeltas from their integer day, second, and microsecond fields. Keep the existing conversion for Pendulum durations and intervals, preserving their current semantics. Integer conversion also avoids losing subsecond precision for large divisors.

The regressions compare all four operations with the standard library using positive, negative, and fractional-second divisors, verify zero-division behavior, exercise interval arithmetic, and check a large divisor whose final microsecond affects the quotient.

Validation:

• Before the change, the 17 initial regression cases failed with AttributeError.
• Pure-Python test suite on Windows / Python 3.14: 1,863 passed, 5 skipped.
• mypy --python-version 3.13, using the repository's locked mypy 1.13.0 and dateutil stubs: passed (228 source files).
• prek run --files src/pendulum/duration.py tests/duration/test_arithmetic.py tests/interval/test_arithmetic.py: all applicable checks passed.
• The Rust extension variant was not run locally; this change is confined to Python duration arithmetic and its tests.

Pull Request Check List

• Added tests for changed code.
• Updated documentation for changed code. No documentation change: this fixes the existing timedelta operand support.

This contribution was prepared with OpenAI Codex assistance.