fix: use last/first colour for over/under when extend=None

[mercael91]

Summary

When extend_levels=False and extend=None, the over and under colours were set to transparent (0, 0, 0, 0). This caused a value exactly equal to levels[-1] to render transparent instead of using the last colour in the map.

Matplotlib's default Colormap behaviour is to fall back to the last colour for the over-range. This PR aligns earthkit-plots with that by defaulting over_color to colors[-1] and under_color to colors[0].

Change

# Before
over_color = (0, 0, 0, 0)
under_color = (0, 0, 0, 0)

# After
over_color = colors[-1]  # Use last colour as over-color
under_color = colors[0]  # Use first colour as under-color

Verification

• A value equal to levels[-1] now renders with the last colour instead of transparent
• A value equal to levels[0] continues to render with the first colour
• Behaviour with extend="both", extend="max", extend="min" is unchanged

Fixes #243