fix: merge TypeInfo for structurally equal types

[sumleo]

Summary

• When collect_equal_types unions structurally identical types from different interfaces, the TypeInfo (borrowed/owned/error flags) was not being merged
• This caused incorrect code generation when a type appeared as an import parameter in one interface and an export return in another — only one side's flags would be visible, leading to missing borrowed or owned type variants
• Added a post-pass in collect_equal_types that merges TypeInfo across all members of each equivalence class using the existing BitOrAssign implementation

Test plan

• Added codegen test in crates/rust/tests/codegen.rs with two interfaces containing structurally equal payload records (one imported, one exported) using merge_structurally_equal_types and Borrowing ownership
• Verified the test compiles and generates code without panic
• All existing workspace tests pass

[alexcrichton]

Thanks! Could the test be modeled as a new file in tests/codegen/*.wit? It could then be modeled here with a new variant that passes --merge-structurally-equal-types