test: add SDK streaming stress examples

[varin-nair-factory]

Summary

Adds a separate stress-test suite for the streaming and structured-output SDK behavior introduced in #37. This keeps the API PR focused while preserving runnable coverage for default message-level streaming, partial chunk-level streaming, result aggregation, tool use, multi-turn behavior, cancellation, and structured-output error paths.

What changed

• Adds examples/stress/* as modular stress scripts that can be run individually or through examples/stress/all.ts.
• Adds examples/structured-output-stress-test.ts for multi-model structured-output validation.
• Writes JSONL artifacts for easier debugging of stream shape and final result payloads.

Validation

• npm run typecheck
• npm run typecheck:examples
• npm run lint
• npm test
• npm run format:check

Stacked on #37.