Add structured JSON helper APIs

[leehack]

Summary

Adds ergonomic structured JSON helpers for issue #126:

• exports LlamaStructuredOutput<T> for JSON object, object-shaped JSON Schema, and value-shaped JSON Schema outputs
• adds final-output parsing and validation, including streamed finalization with parseStructuredJson(...)
• adds LlamaEngine.createStructuredJson(...) for typed structured JSON calls
• documents usage in README and website generation guides

Fixes #126.

Validation

• dart test test/unit/core/models/inference/structured_output_test.dart passed on VM and Chrome
• dart test test/unit/core/engine/engine_test.dart passed on VM and Chrome
• dart analyze lib/src/core/models/inference/structured_output.dart test/unit/core/models/inference/structured_output_test.dart passed
• git diff --check passed
• Direct real-model E2E passed:
  • dart run .dart_tool/e2e/structured_output_real_model_smoke.dart /opt/UnitySrc/personal/llama/llamadart/models/qwen2.5-0.5b-instruct-q4_k_m.gguf
  • result: RESULT structured_output_real_model {"answer":"ok","count":3}

Notes

Full root dart analyze is currently blocked by unrelated existing example package setup errors around example/basic_app imports/dependencies. The touched files analyze cleanly.

[github-actions]

Chat app preview removed for leehack/llamadart-chat-pr-265.

[Copilot]

Pull request overview

Adds a new public API layer for “structured JSON output” generation, providing a typed helper (LlamaStructuredOutput<T>) plus an engine convenience (LlamaEngine.createStructuredJson(...)) that finalizes a streamed response by parsing/validating JSON and then decoding into an application type. This aligns with issue #126’s goal of making strict JSON / JSON Schema outputs ergonomic and safer.

Changes:

• Added LlamaStructuredOutput<T> with JSON-object and JSON-schema helpers plus final-output parsing/validation and a stream finalization extension (parseStructuredJson).
• Added LlamaEngine.createStructuredJson(...) to apply structured response formats and decode the final validated value.
• Updated README + website guides and added changelog entries; added unit tests for schema validation, malformed output, streaming finalization, and engine integration.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
website/docs/guides/generation-and-streaming.md	Adds a “Structured JSON output” guide section and usage example.
website/docs/guides/chat-template-and-parsing.md	Updates guidance for responseFormat and structured JSON helpers.
website/docs/changelog/recent-releases.md	Adds an Unreleased entry for structured JSON helpers.
test/unit/core/models/inference/structured_output_test.dart	Adds unit tests for schema validation, parsing failures, and streaming finalization.
test/unit/core/engine/engine_test.dart	Adds test coverage for createStructuredJson forwarding grammar + decoding output.
README.md	Documents structured JSON output usage and updates section numbering.
lib/src/core/models/inference/structured_output.dart	Implements LlamaStructuredOutput<T>, schema subset validation, output validation, and stream parsing extension.
lib/src/core/engine/engine.dart	Adds createStructuredJson(...) and references structured output helpers in docs.
lib/llamadart.dart	Exports the new structured output API.
CHANGELOG.md	Adds an Unreleased entry describing the new helpers.

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