Fix pending interaction queue send guard

[ymichael]

Summary

• reject /threads/:id/send queue-if-active requests while an active thread is awaiting user interaction
• add a public API regression proving no queued message is created in that pending-interaction state
• stabilize two full-suite tests observed during the QA pass and record the manual runbook results

Validation

• pnpm exec turbo run lint
• pnpm exec turbo run typecheck
• pnpm exec turbo run test
• pnpm exec turbo run test:integration
• pnpm exec turbo run build --filter=@bb/scripts --filter=@bb/app --filter=@bb/server --filter=@bb/host-daemon --filter=@bb/cli --filter=bb-app
• qa/manual-runbook.md standalone pass, including real Codex, Claude Code, and Pi provider probes

Notes

• raw local QA logs were captured under qa/logs/2026-06-12-full-qa/ but are intentionally not committed