Simplify validation for Case IR node

[electrum]

Release notes

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[Copilot]

Pull Request Overview

This PR simplifies the validation logic for the Case IR node by consolidating type validation checks. Instead of separately validating that all when clauses have the same result type and that the default value matches the first when clause's type, it now validates that all when clause results match the default value's type.

• Consolidated type validation logic by comparing all when clause results against the default value type
• Added argument validation to ensure when clauses are not empty
• Removed redundant validation loops

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