feat: add structured output (JSON schema) support

Add outputFormat option to session.prompt() for requesting structured JSON
output. When type is 'json_schema', injects a StructuredOutput tool that
validates model output against the provided schema.

- Add OutputFormat schema types (text, json_schema) to message-v2.ts
- Add structured_output field to AssistantMessage
- Add StructuredOutputError for validation failures
- Implement createStructuredOutputTool helper in prompt.ts
- Integrate structured output into agent loop with retry support
- Regenerate OpenAPI spec with new types
- Add unit tests for schema validation