Add RGBA color support to custom colormaps

[dcherian]

Summary

This PR adds support for RGBA hex colors (#RRGGBBAA) in custom colormaps while maintaining full backward compatibility with existing RGB (#RRGGBB) colormaps.

Changes

• Validator: Updated validate_colormap() to accept both #RRGGBB and #RRGGBBAA formats
• Tests: Added comprehensive unit tests for RGBA validation (mixed RGB/RGBA, transparency levels, invalid formats)
• Integration Test: Added test with categorical data using RGBA colormap with fully transparent category
• Testing Library: Added helper functions assert_has_transparent_pixels() and assert_has_opaque_pixels()
• Documentation: Updated README with RGBA support examples and tips
• Type Hints: Updated field descriptions in TileQuery and WMSGetMapQuery to document RGBA support
• Comments: Added note about matplotlib's native RGBA support in LinearSegmentedColormap

Backward Compatibility

✅ Fully backward compatible - all existing RGB-only colormaps continue to work without changes

Test Results

• All existing colormap tests pass
• 6 new RGBA-specific tests added and passing
• New snapshot test verifies alpha channel rendering works correctly
• Pre-commit checks pass

Example Usage

# Continuous data with transparency gradient
colormap = {
    "0": "#0000ffff",    # Blue, fully opaque
    "128": "#00ff0080",  # Green, 50% transparent  
    "255": "#ff000000"   # Red, fully transparent
}

# Categorical data with one transparent category
colormap = {
    "0": "#ff000000",  # Transparent
    "1": "#00ff00ff",  # Green, opaque
    "2": "#0000ffff"   # Blue, opaque
}

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.45%. Comparing base (9afa5fe) to head (5aa984c).

Additional details and impacted files

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  Coverage   75.45%   75.45%           
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  Files          26       26           
  Lines        3541     3541           
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  Hits         2672     2672           
  Misses        869      869           

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[dcherian]

This is blocked by holoviz/datashader#1404