fix: exclude internal style cols from tooltips

[Azaya89]

fixes #391

This PR:

• Fixes hover tooltips showing confusing internal _color and _size fields when passing mapped Series to color/size parameters. The fix automatically excludes these internal columns from default hover tooltips while respecting explicit hover_tooltips configurations.

• Documentation updated to recommend using color='column' + cmap={...} for custom color mapping.

• Added tests

Examples

1. From Issue Specifying color adds _color entry hover-over #391

import hvplot.pandas

penguins = hvplot.sampledata.penguins('pandas')
penguins.hvplot.scatter(
    x="bill_length_mm",
    y="bill_depth_mm",
    color=penguins["species"].map({
        "Adelie":"blue",
        "Gentoo":"yellow",
        "Chinstrap":"red",
    }),
)

2. Recommended approach

import hvplot.pandas

penguins = hvplot.sampledata.penguins('pandas')
penguins.hvplot.scatter(
    x="bill_length_mm",
    y="bill_depth_mm",
    color='species',
    cmap={
        "Adelie":"blue",
        "Gentoo":"yellow",
        "Chinstrap":"red",
    },
)