Fix splitting issue with pvgridder.extract_boundary_polygons() when a polygon has more than 1000 edges

[keurfonluu]

• Fixed: allow extracted polygons to have more than 1000 edges by setting option max_length to the number of points. Addresses issue raised in The line for a boundary has no order when extracting the points pyvista/pyvista#7759 (comment).

Reminders:

• Run invoke ruff to make sure the code follows the style guide,
• Add tests for new features or tests that would have caught the bug that you're fixing,
• Write detailed docstrings for all functions, classes and/or methods,
• If adding new functionality, unit test it and add it to the documentation.