fix: Optimize code block rendering in streaming mode to prevent unnecessary re-renders

[WTW0313]

Description

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Performance improvement
• Refactoring (no functional changes)

Related Issues

Fixes #435

Changes Made

• Extract the inline default values for linkSafety into module-level constants (defaultLinkSafetyConfig)

Testing

• All existing tests pass
• Added new tests for the changes
• Manually tested the changes

Test Coverage

Screenshots/Demos

streamdown-demo.mp4

Checklist

• My code follows the project's code style
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings or errors
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have created a changeset (pnpm changeset)

Changeset

• I have created a changeset for these changes

Additional Notes

[vercel]

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