Ignoring later fields instead of overriding when getting dup fields. - #10053

Summary

When a struct has duplicate member names, the first declaration is now preserved instead of being overwritten by the last one. This matches the existing behavior for enum variant redefinition. Additionally, a test expectation for member a's type was corrected from () to core::felt252.

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 struct member was redefined (declared more than once), the map insertion via insert would overwrite the first declaration with the duplicate. This meant the type and visibility of the last duplicate were retained, even though the duplicate is the one being reported as an error and rejected. This is inconsistent with how enum variant redefinitions are handled.

What was the behavior or documentation before?

On a StructMemberRedefinition error, the last duplicate member's id, ty, and visibility were stored in the members map, discarding the original declaration.

What is the behavior or documentation after?

On a StructMemberRedefinition error, the first (original) member declaration is kept in the members map. The duplicate triggers the diagnostic and is discarded, consistent with enum variant redefinition handling.

Related issue or discussion (if any)

N/A

Additional context

The Entry-based approach (Entry::Vacant / Entry::Occupied) replaces the previous insert-and-check pattern to avoid overwriting the original entry on collision.