test: cover design-system color and prefix validators

[Dennis-Ritchie1]

test: cover design-system color and prefix validators

Closes #333

📌 Summary

Adds focused unit-test coverage for the low-level string validators in design-system/src/utils/validators.ts. The existing validators.test.ts covered
isValidColorToken/isValidChartTokens but only lightly touched the primitive validators. This PR adds dedicated boundary tables for them.

🎯 What's covered

• isValidHexColor — accepts #0A7668; rejects 3-digit (#abc), 8-digit (#0A7668FF), missing #, non-hex chars, empty/whitespace, surrounding whitespace.
• isValidRgbColor — accepts canonical forms; rejects extra spaces/commas, missing channel, rgba() alpha variant, empty/whitespace.
• isValidHslColor — accepts canonical forms; rejects missing %, hsla() alpha variant, extra spaces, empty/whitespace.
• isKebabCase — accepts chart-grid; rejects Chart-Grid (mixed case), chart_grid (underscore), 1chart (leading digit), chart- (trailing hyphen),
  empty/whitespace.
• hasValidTokenPrefix — accepts each documented prefix (color-, spacing-, typography-, shadow-, radius-, border-, motion-); rejects unknown prefixes
  (chart-foo) and prefix-without-hyphen (color, colorfoo), empty/whitespace.

✅ Results

• cd design-system && npm test → 53 passed, 5 suites, no regressions.
• Coverage on validators.ts: 100% statements / branches / functions / lines (target was 95%).

📂 Changes

• design-system/src/tests/validators.test.ts — appended new boundary-table describe blocks (test-only change; no production code modified).

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[1nonlypiece]

good to have the color and prefix validators pinned down by tests. merged