fix: preserve response input message roundtrips

[SiluPanda]

Closes #584.

ResponseInputItem.Message.Builder left its documented constant type unset, so serializing a user-built message omitted the union discriminator and deserializing it produced _unknown. EasyInputMessage.Builder had the same missing default.

This change:

• defaults both message builders to type: "message"
• makes structured, non-assistant message JSON without phase deserialize to the narrower ResponseInputItem.Message variant
• preserves EasyInputMessage preference for string content, assistant roles, phase-bearing messages, missing roles, and unknown/future roles
• adds roundtrip and discriminator-boundary coverage

The tie-break is necessary because a structured EasyInputMessage and Message with a user, system, or developer role and no phase are wire-identical. The new test documents Message as the canonical branch for that shape rather than leaving the choice implicit.

Validation

• ./gradlew :openai-java-core:test --tests com.openai.models.responses.ResponseInputItemTest --tests com.openai.models.responses.EasyInputMessageTest :openai-java-core:lintKotlin --console=plain
  • ResponseInputItemTest: 73 tests, 0 failures
  • EasyInputMessageTest: 2 tests, 0 failures
  • Kotlin lint passed
• authoritative local Castiron budget check against current main: 1,735 / 2,000 custom lines, with budget isolation and budget checks both passing
• git diff --check

Security review note

This touches Jackson polymorphic deserialization. The added dispatch condition only examines the already-parsed type, role, content shape, and phase presence; it adds no I/O, recursion, payload limit, or credential/logging behavior. Exact known input roles are required before changing the existing ordering, so malformed, missing, and future roles retain the current EasyInputMessage-first path.

AI assistance: I used Codex to help investigate the generated union behavior, draft the implementation and tests, and run validation. I reviewed the resulting code, the ambiguity tradeoff, and the test output.

[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 2026-08-30T12:52:26.555724Z	c04f0c7	PR opened
🔒 Security Review	✅ Completed 2026-08-30T12:53:04.748025Z	c04f0c7	PR opened

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[sylvesterkaczmarek]

Went through the ambiguous type: "message" branch. The builder defaults fix the missing discriminator, and the tie-break is consistent: structured user/system/developer messages without phase go to Message, while string/assistant/phase shapes stay EasyInputMessage. Since the ambiguous wire shape is identical either way, pinning one canonical branch in tests seems reasonable. No blocker from me.

[SiluPanda]

@dpiet-oai, gentle follow-up: the current response-message round-trip fix has received positive code-review feedback; could you take a look when convenient? Thank you!