Emit LLM stream lifecycle events

[kitlangton]

Summary

• emit step/text/reasoning lifecycle events from native LLM protocol streams
• teach tool-stream helpers to emit tool input start/end events consistently
• update provider and tool-runtime tests to enforce lifecycle event ordering

Verification

• cd packages/llm && bun typecheck
• cd packages/llm && bun run test -- test/tool-runtime.test.ts test/tool-stream.test.ts test/provider/openai-chat.test.ts test/provider/openai-compatible-chat.test.ts test/provider/openrouter.test.ts test/provider/openai-responses.test.ts test/provider/anthropic-messages.test.ts test/provider/gemini.test.ts test/provider/bedrock-converse.test.ts
• bunx prettier --check
• git diff --check HEAD~1..HEAD