Add typed metadata property builder

[YuqiGuo105]

Summary

• add a typed Metadata.Builder.putProperty(String, String) helper for Responses
• expose the same helper for the Beta Responses API
• preserve putAdditionalProperty(String, JsonValue) as the escape hatch for undocumented values
• add regression tests that verify string values are stored as JSON strings

Motivation

The API only accepts string values for metadata, but the existing builder exposes only the raw JsonValue method. This allows values such as JSON numbers to compile and fail later with a 400 response. The typed helper provides a straightforward, API-aligned path while keeping the existing low-level method backward compatible.

Testing

• targeted stable and Beta ResponseCreateParamsTest tests
• :openai-java-core:lintKotlin
• full offline build with mock-server-dependent tests skipped
• Castiron custom-code budget check (1886 / 2000)

Closes #338

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-02T18:30:10.986425Z	1790828	PR opened
🔒 Security Review	✅ Completed 2026-09-02T18:29:28.640336Z	1790828	PR opened

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[sylvesterkaczmarek]

This is a useful API-level narrowing over the raw additional-properties escape hatch. The helper guarantees metadata values are serialized as JSON strings, which matches the service contract and avoids callers accidentally sending numeric JSON for numeric-looking IDs. Mirroring it in both stable and beta response params keeps the two surfaces consistent.