feat: add support for hexadecimal RGBA color format

[aza-ali]

Summary

Adds support for 4-digit and 8-digit hexadecimal color codes with alpha channel to the Color class, addressing issue #1849.

Users can now use colors like:

• #FF00FF80 (50% transparent magenta)
• #f0f8 (shorthand 4-digit RGBA)

Changes Made

Core Functionality

• Extended Color dataclass with optional a field (alpha channel, default 255)
• Updated _validate_color_hex() to accept lengths 3, 4, 6, and 8
• Modified from_hex() to parse 4-digit (#RGBA) and 8-digit (#RRGGBBAA) hex codes
• Updated as_hex() to return #RRGGBBAA format when alpha != 255, otherwise #RRGGBB

New Methods

• as_rgba() - returns RGBA tuple
• as_bgra() - returns BGRA tuple
• from_rgba_tuple() - creates Color from RGBA tuple
• from_bgra_tuple() - creates Color from BGRA tuple

Internal Updates

• Updated __eq__() and __hash__() to include alpha channel
• Comprehensive docstrings with examples for all new functionality

Testing

• Added 27 new unit tests covering:
  • Parsing 4-digit and 8-digit hex codes with alpha
  • Converting to/from RGBA and BGRA tuples
  • Round-trip conversions
  • Edge cases (alpha=0, alpha=255)
  • Invalid inputs
• All 795 tests pass successfully
• Pre-commit hooks pass (ruff, bandit, codespell, etc.)

Backward Compatibility

✅ Fully backward compatible - all existing code continues to work:

• Default alpha value is 255 (fully opaque)
• Existing 3-digit and 6-digit hex codes work as before
• as_hex() returns 6-digit format when alpha=255
• Public API remains unchanged

Resolves

Closes #1849

[CLAassistant]

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