feat(generation): add structured JSON generation helpers

[leehack]

Feature description

Add high-level helpers for structured JSON generation, building on the existing grammar/GBNF capabilities.

Motivation

Many app integrations need reliable machine-readable output. Users should not have to manually write grammars or prompt wrappers for common JSON-object / JSON-schema use cases.

Proposed scope

• Provide an ergonomic API for strict JSON-object generation.
• Consider JSON Schema to grammar/GBNF conversion for a practical MVP subset.
• Add validation and helpful errors when a schema cannot be represented safely.
• Make the helper compatible with streaming where possible, or clearly document limitations.
• Include examples for extraction/classification-style app tasks.

Related work

• Builds on the completed grammar sampling work tracked in Feature: Grammar Sampling (GBNF) #10.

Acceptance criteria

• Public API docs explain supported schema features and limitations.
• Examples demonstrate generating typed JSON and validating the final output.
• Tests cover success, invalid schema, and malformed model-output cases.