bugfix(semantic): Removed extra diagnostic for type mismatches on missing. - #10121

Summary

When an impl constant references an unresolvable type (e.g., const X: Nonexistent = 0;), the compiler was previously emitting both a "Type not found" error and a spurious "WrongType" mismatch diagnostic. The type mismatch check in validate_impl_item_constant now skips reporting when either the expected or actual type is a missing (error sentinel) type, suppressing the cascading false diagnostic.

Type of change

Please check one:

• Bug fix (fixes incorrect behavior)
• New feature
• Performance improvement
• Documentation change with concrete technical impact
• Style, wording, formatting, or typo-only change

Why is this change needed?

When a type in an impl constant declaration could not be resolved, the compiler would report the legitimate "Type not found" error and then also emit a spurious "WrongType" diagnostic, because the missing type sentinel did not match the expected concrete type. This caused confusing double-error output for a single root cause.

What was the behavior or documentation before?

Given:

trait MyTrait {
    const X: u32;
}
impl MyImpl of MyTrait {
    const X: Nonexistent = 0;
}

The compiler emitted both a "Type not found" error for Nonexistent and a secondary "WrongType" mismatch error.

What is the behavior or documentation after?

The compiler now emits only the "Type not found" error, suppressing the cascading type-mismatch diagnostic when either type involved in the comparison is a missing/error sentinel type.

Related issue or discussion (if any)

N/A

Additional context

A new test case was added to trait_const to assert that only the resolution error is reported and no spurious type-mismatch diagnostic appears.