Add issue #374 regression coverage for C# object initializers

[Widthdom]

Summary

• add an extractor-level regression test for the issue C# enum member regex over-matches object initializer assignments (Age = 30, inside new Person { } becomes phantom function) #374 object-initializer repro
• add a CLI integration test that verifies symbols does not surface phantom Age / Priority entries after indexing
• document the regression coverage in the changelog

Verification

• dotnet build
• dotnet test
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll . --json
• adversarial review via codex exec --ephemeral ... with no blocking/actionable findings

Closes #374.