feat(model-json): streaming projecting + validating JSON converter

[jfallows]

Description

Surfaces the streaming, single-pass projecting + validating path as the model-json converter, built on the common-json pipeline primitives (#1855 / #1846).

The read/write converters are re-platformed onto the common-json streaming pipeline parser → schema.validator() → projector(schema) → generator, producing a compact, schema-pruned, validated document into a MutableDirectBuffer — no full-document DOM, no per-call builder allocation. Projection pointers are derived from the schema via JsonSchema.retainedPaths(). The JsonSchema is compiled and cached per schemaId; the pipeline is cached per schema.

Per maintainer direction, this lands the converter layer as streaming + projecting and keeps the validator handler streaming validate-only for now; the converter/validator unification (single fragment-aware convert) is tracked by #1836. The converter currently receives the whole value in one convert(...) call, so the pipeline is driven with a single feed(..., last=true).

Failure model — streaming-validate-then-abort

On a late validation failure the pipeline reports REJECTED; the converter runs a diagnostic pass to surface the schema diagnostic via validationFailure and returns -1, so the binding aborts the downstream stream (engine RESET) rather than buffering to reject before forwarding. EventIT exercises this end-to-end and still asserts the exact diagnostic ([1,11][/id] expected string but was number).

Changes

• JsonModelHandler — per-schemaId JsonSchema cache + per-schema pipeline cache; project(...) drives the pipeline into a grow-on-demand buffer (wrapped at full capacity with headroom, since the sink suspends when remaining() < 16); diagnostic pass on rejection; field extraction retained as a streaming pass over the input.
• JsonReadConverterHandler / JsonWriteConverterHandler — emit the projected document (read forwards the projected buffer then extracts; write encodes the projected buffer).
• JsonValidatorHandler — unchanged (already streaming validate-only).
• JsonConverterTest — updated test-first to assert the projected (compacted, pruned) output and lengths.

Scope / compatibility

Isolated to model-json: no other module routes model: json through supplyReadConverter / supplyWriteConverter (e.g. binding-mcp-http resolves schema text itself for its own projector), so the change in forwarded bytes does not affect other bindings.

Tests

./mvnw verify -pl runtime/model-json — BUILD SUCCESS: 16 unit tests + EventIT green; checkstyle, license, and the JaCoCo coverage gate all pass.

Fixes #1835

🤖 Generated with Claude Code

---

Generated by Claude Code