fix: improve solhint custom rules and remove TODOs

[crStiv]

This commit improves the solhint custom rules by:

1. Expanding visibility check for constant variables to include internal constants
2. Re-enabling and fixing the no-external-virtual rule with:
   • Skip checks for constructors and interfaces
   • Improved error message with actionable suggestion
   • Fixed comparison operator

Both TODO comments have been addressed while maintaining the codebase's
coding standards and improving the overall code quality checks.

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Latest commit: 1589458

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