Design a dependency-free four-state JSON field model for generated types

[OmarAlJarrah]

Problem

A generator needs a field wrapper distinguishing present-typed, present-but-wrong-type, explicit-null, and absent — and round-tripping unknown shapes. Tristate covers three states (absent/null/present) but has no raw/wrong-type escape hatch and no embedded JSON tree.

Proposed change

Design a dep-free sealed JsonField<T> (Known<T:Any> / Missing / Null / Raw(RawJson)) plus a RawJson tree (object/array/string/number/bool/null) in sdk-core, with all Jackson↔RawJson conversion behind the Serde SPI in the adapter — mirror how Tristate is split. Map Tristate onto it. Kotlin sealed class compiles to Java-8 bytecode (no permits).

Prior art: openai-java's JsonField/JsonValue (Values.kt) — but it welds Jackson onto the value type; we must not. Reuse the contextual-type-extraction trick from our TristateModule.

Acceptance

• Dep-free JsonField + RawJson in core
• Jackson conversion in adapter only
• Tristate interop documented, incl. the read-path (forward-compat) vs PATCH-path (three-state) boundary so the two are never wrongly merged
• apiDump

Foundation for the rest of the codegen items.

Dependencies

• Blocks Generate thin model classes over a hand-written runtime #51, additionalProperties pass-through on generated models #52, Generate unions as private-ctor + per-variant accessors + visitor (Java-8 safe) #53, Generate forward-compatible enums (open value + known/value pair) #54, Generate an optional validate()/isValid()/validity() triad on models #64, Generate discriminator/const fields as defaulted raw values with dual accessors #65 — all generated-model work builds on the four-state field model + RawJson tree
• Foundation for the codegen track

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Priority: highest · Effort: large