Fix Pi extension-triggered turn completion

[ryanbbrown]

Fixes #1633.

Summary

• accept string content in Pi conversation messages, as allowed by AgentMessage
• complete extension-triggered turns when agent_end contains a custom message with string content
• cover the pi-processes notification shape with a regression test

Root cause

The Pi adapter accepted only content-block arrays in agent_end.messages. Pi extensions can also send user or custom messages with string content. Schema validation rejected the complete event, emitted provider/unhandled, and prevented turn/completed for extension-triggered turns.

Validation

• pnpm exec turbo run test typecheck --filter=@bb/agent-runtime --force
  • 46 test files passed
  • 954 tests passed
  • typecheck passed
• git diff --check

AGENT GENERATED: by GPT-5.6 Sol