Fix type hints for interval and span_range to accept Arrow objects

[rstar327]

Pull Request Checklist

• 🧪 Added tests for changed code.
• 🛠️ All tests pass when run locally (run tox or make test to find out!).
• 🧹 All linting checks pass when run locally (run tox -e lint or make lint to find out!).
• 📚 Updated documentation for changed code.
• ⏩ Code is up-to-date with the master branch.

Description of Changes

Arrow.interval and Arrow.span_range accept Arrow objects for start and end parameters, but the type hints only specified datetime.datetime. This causes false type errors in type checkers like mypy/pyright.

Updated type hints from dt_datetime to Union["Arrow", dt_datetime] for both methods, matching the existing pattern used in Arrow.range.

Added test_arrow_objects_as_start_end to verify Arrow objects work correctly with interval. All 1905 tests pass.

Closes: #1210

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (b423717) to head (84d04f6).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #1255   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           10        10           
  Lines         2315      2315           
  Branches       358       358           
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  Hits          2315      2315           

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