Fix duplicated arrow tips in DashedVMobject (issue #3220)

[jakekinchen]

Summary

• Prevent DashedVMobject from duplicating tips: copy the input, pop tips once, build dashes from the tipless copy, then reattach the original tips a single time.
• Keeps opacity consistent when groups set fill/stroke with family=True; works with default and custom tip shapes and with start+end tips.
• Adds regression tests covering dashed arrows (default, custom tip, start+end tip) and group opacity to guard against stacked tips.

Testing

• uv run pytest tests/module/mobject/types/vectorized_mobject/test_dashed_vmobject.py -q
• uv run pytest

[behackl]

Thank you very much for your contribution, this looks fine to me!