refactor(mocks): extract MockTypeModel.Visibility helper

[thomhurst]

Summary

• Follow-up cleanup to [Bug]: CS9338 for interface mock when interface is not public #5426 fix (commit 53bd350 on main).
• Extracts a single Visibility property on MockTypeModel to replace the duplicated IsPublic ? "public" : "internal" ternary across five builders.
• Restores the optional defaults on spanReturnElementType / spanReturnType in GenerateVoidUnifiedClass by moving the visibility parameter earlier in the signature (it had been demoted to required to make room for the new param).

Test plan

• TUnit.Mocks.Tests builds clean
• Issue5426Tests (3/3) pass

[claude]

Code Review

Clean, well-scoped refactor. No issues found.

What this does well

DRY principle applied correctly — the duplicated model.IsPublic ? "public" : "internal" ternary appeared in 5 separate builders, all computing the same value from the same source. Extracting it to MockTypeModel.Visibility is the right place for this logic: the model owns the data, so the derived string belongs there too.

Signature fix in GenerateVoidUnifiedClass — moving visibility to an earlier position in the parameter list was necessary to restore the optional defaults on spanReturnElementType / spanReturnType. C# requires that parameters with defaults trail all required parameters, so the previous ordering was forcing callers to always pass nulls explicitly. The reordering is correct and improves call-site ergonomics.

Minimal footprint — the PR touches exactly the right files and nothing more. The XML doc comment on Visibility is concise and useful.

Minor notes (no action needed)

• IsPublic is still present on the model (correctly, as the source of truth). Visibility is a purely derived helper — that separation is appropriate.
• If visibility ever needs to support more than two states (e.g., protected internal), this is the one place to update — a good consolidation point.

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
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