Fix hex color decorators for 3-digit and 4-digit hex colors

[Copilot]

Hex colors were not showing color preview decorators for 3-digit (#RGB) and 4-digit (#RGBA) hex formats, even when properly quoted in strings. The regex pattern in defaultDocumentColorsComputer.ts only supported 6-digit (#RRGGBB) and 8-digit (#RRGGBBAA) hex colors.

Before

contentPrimary: '#FFFFFF',     // ✅ Shows color decorator (6-digit)
contentSecondary: '#abc',      // ❌ No color decorator (3-digit) 
contentTertiary: '#abcd',      // ❌ No color decorator (4-digit)
contentError: '#ED4964AB',     // ✅ Shows color decorator (8-digit)

After

contentPrimary: '#FFFFFF',     // ✅ Shows color decorator (6-digit)
contentSecondary: '#abc',      // ✅ Shows color decorator (3-digit) 
contentTertiary: '#abcd',      // ✅ Shows color decorator (4-digit)
contentError: '#ED4964AB',     // ✅ Shows color decorator (8-digit)

Changes Made

1. Updated regex pattern in defaultDocumentColorsComputer.ts to include:

   • 3-digit hex colors: #RGB (expands to #RRGGBB)
   • 4-digit hex colors: #RGBA (expands to #RRGGBBAA)

2. Added comprehensive test cases covering all hex color formats

3. Maintained security measures - preserved existing whitespace/quote prefix requirements to prevent GitHub issue numbers from being detected as colors

Security Considerations

The fix maintains the existing security behavior:

• ✅ background: '#abc' - Shows decorator (quoted)
• ✅ color: #abc - Shows decorator (space prefix)
• ❌ issue#abc123 - No decorator (no space/quote prefix)

The Color.Format.CSS.parseHex function already supports 3-digit and 4-digit hex parsing, so no changes were needed to the color parsing logic.

Fixes #239446.

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