fix(cli): remove automatic color contrast adjustment warnings

[devin-ai-integration]

Description

Refs: Customer request from Jeff Reiner via Slack

Removes the misleading warning messages that claimed "Fern will adjust the color to meet the minimum contrast ratio" during docs validation. These warnings were confusing because the CLI doesn't actually adjust colors - the color palette generation happens on the frontend.

Changes Made

• Removed warning messages for contrast ratios between 3:1 and 7:1 that incorrectly stated Fern would adjust colors
• Removed unused helper functions (getDesiredRatio, getUserReadableRatio, increaseForegroundContrast) that were never called
• Kept the warning for contrast ratios below 3:1 (minimum for UI components) as this is a genuine accessibility concern

Testing

• Manual testing completed - verified lint passes
• Unit tests added/updated

Human Review Checklist

• Verify this aligns with the user's intent - they wanted to disable the "automatic color changes" warnings
• Confirm the remaining 3:1 contrast warning is still appropriate to keep

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Requested by: Deep Singhvi (@dsinghvi)
Link to Devin run: https://app.devin.ai/sessions/106f8d6c624946349b23a34c147e5a28

[devin-ai-integration]

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