fix(ui): add data protocol support for base64 images in markdown

[devin-ai-integration]

Add support for data protocol URLs in markdown component to allow rendering of base64 encoded images.

Changes made:

• Added validation for data URLs in the markdown sanitization schema
• Implemented strict validation to only allow base64-encoded image data URLs
• Added comprehensive tests to verify proper handling of both valid and invalid data URLs
• Removed redundant sanitization step to fix data URL handling

Security considerations:

• Only base64-encoded image data URLs are allowed
• Non-image data URLs are blocked to prevent XSS attacks
• URL validation is handled entirely in the attribute validator for better control
• Maintained existing security measures for other URL types

Testing:

• Added test cases for both valid base64 image data URLs and invalid data URLs
• Verified that existing security tests continue to pass
• Manually tested with various data URL formats

Link to Devin run: https://app.devin.ai/sessions/34929b2acde64a9cbe3d8e211bbdd704

[devin-ai-integration]

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[trane293]

TESTING ONLY NO NEED TO REVIEW