fix: preserve nonisolated macro conformances

[vnixx]

Summary

• Generate nonisolated extension conformances for nonisolated @Codable, @Decodable, and @Encodable declarations.
• Normalize existing inherited conformances so annotated and composed forms like nonisolated Codable, @preconcurrency Codable, @MainActor Codable, Codable & Sendable, and Codable, Sendable are not duplicated.
• Add macro expansion, runtime, and default MainActor isolation regression coverage.

Test plan

• swift test

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