[patch] Reject non-hex characters in Color.FromHex with ArgumentException

[matt-edmondson]

Fixes #313

What was wrong

Color.FromHex checked only the string's length, then parsed each channel with Convert.ToByte(s, 16). That call has two quirks:

• It accepts a leading +, so #+F+F+F was silently accepted as #0F0F0F.
• It throws FormatException for most other bad characters (#GGGGGG, #0x0x0x) and ArgumentException for -. The exception type depended on which bad character showed up, and the docs promise only ArgumentException.

Change (Semantics.Color/Color.Conversions.cs)

• After the length check (and 3-digit expansion), every character must be 0-9, A-F or a-f. Anything else throws ArgumentException(..., nameof(hex)), which names the bad character.
• Convert.ToByte now only ever sees hex digits, so it can no longer accept a sign or throw FormatException. I kept it rather than switching to byte.Parse(ReadOnlySpan<char>, …) because that overload isn't available on the netstandard2.0 target.
• Updated the <exception> doc to cover non-hex characters.

Tests (HexConversionTests)

• FromHex_NonHexCharacter_ThrowsArgumentException: #+F+F+F, +F+F+F, #00+F00, #GGGGGG, #0x0x0x, #-1-1-1, #12345G, #FFFFFF+F, #F F, and #G00 all throw exactly ArgumentException.
• FromHex_ValidDigitsInEitherCase_RoundTrip: valid 3, 6 and 8-digit inputs in lower and upper case, with and without #, still round-trip.

With the fix reverted, 9 of these cases fail. With the fix, the full Semantics.Test suite passes: 1495 passed, 0 failed, 8 skipped. Semantics.Color builds for every target framework.

🤖 Generated with Claude Code

https://claude.ai/code/session_019RYN9jT6szUiySkywFKCA1

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Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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