Generate forward-compatible enums (open value + known/value pair)

[OmarAlJarrah]

Problem

Closed enums throw on an unrecognised server value.

Proposed change

Generate enums as an open class over the four-state string field, with a closed Known enum and a Value enum that adds an unknown sentinel. Deserialization never throws; a known()/value() accessor throws on unknown; the raw string is preserved. Lint-clean names (no underscore-prefixed sentinel).

Prior art: openai-java's two-enum pattern (ReasoningEffort.kt). Realises the forward-compatible-enum direction already noted in docs/refs-comparison.md.

Acceptance

• Enum codegen template
• Unknown value deserializes without throwing
• Test

Dependencies

• Blocked by Design a dependency-free four-state JSON field model for generated types #50 — open value over the four-state string field

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Priority: high · Effort: medium