Preserve native LLM tool context

[kitlangton]

Summary

• pass tool call id/name into @opencode-ai/llm tool execute handlers
• allow LLMEvent.stepFinish / requestFinish constructors to accept plain usage input
• cover both behaviors with focused package tests

Verification

• cd packages/llm && bun typecheck
• cd packages/llm && bun run test -- test/schema.test.ts test/tool-runtime.test.ts test/generate-object.test.ts
• prettier check on changed package files
• git diff --check
• pre-push root bun turbo typecheck