feat(llm): add response reducer

[nexxeln]

Summary

• add the canonical LLMResponse reducer for assembling streamed events into assistant messages
• make generate fold through the same reducer and require a terminal finish/provider-error
• add reducer coverage for interleaving, partial streams, and tool-call assembly

Tests

• cd packages/llm && bun typecheck
• cd packages/llm && bun test
• pre-push hook passed: bun turbo typecheck (29/29 tasks)