Improve default argument type mismatch errors

[cknitt]

Summary

Improve the type error shown for optional arguments with invalid default values. Fixes #8387.

Optional argument defaults are lowered through a generated match, which previously caused type mismatches in the default value to be reported as if they came from a user-written switch. This tags the generated optional-default match internally and uses function-style diagnostics for that path instead.

Changes

• Mark generated optional-default match expressions with an internal #optional_arg_default attribute.
• Treat that generated match as function context when selecting type mismatch wording.
• Add a super-errors fixture for an optional default argument type mismatch.
• Regenerate super-errors snapshots.

Note

The snapshot regeneration also produced unrelated whitespace-only changes in a few existing expected files.

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commit: 879c8a5