filler plugin fill extends past line when offset: true + index scale min/max

[sarathkumarsasi]

Fixes #12280

Root cause: LineController restricts the line's stroke to the index range implied by scales.x.min/max (via _getStartAndCountOfVisiblePoints), but the filler plugin only clipped the fill to the chart's rectangular chartArea, not to that same index-scale range. With offset: true, an out-of-range point's pixel position can still land inside chartArea, so the fill shape kept extending past the point where the line stroke stopped.

Fix: _drawfill now additionally clips to the index scale's min/max pixel bounds (matching what bounds the line stroke), skipping this for radial/circular scales (e.g. radar) where a linear pixel bound doesn't apply.

Example:
Bundle with issue: https://codepen.io/sarathkumarps/pen/BypbpBa
Bundle with the fix: https://codepen.io/sarathkumarps/pen/pvRYNmE

[sarathkumarsasi]

@kurkle, @etimberg, please review when you guys get some time.

[etimberg]

Would be good to include a test for this as well

[sarathkumarsasi]

Would be good to include a test for this as well

Added an image-based test fixture

[sarathkumarsasi]

Why is the workflow still pending after the approval?
@etimberg @kurkle

[etimberg]

Why is the workflow still pending after the approval? @etimberg @kurkle

I think it needed to be reapproved after changes. I did that now