Fix Windows Pi shim spawning

[backnotprop]

Summary

• resolve Windows npm command shims before spawning Pi from both Bun and Node providers
• handle Pi Node spawn failures without crashing the Pi extension server
• add a Windows CI/release smoke for the Pi extension AI runtime with an extensionless npm-style pi shim

Verification

• bun test
• bun test packages/ai/ai.test.ts
• bun build scripts/smoke-pi-extension-ai-runtime.ts --target=node --outfile /tmp/pi-ai-runtime-smoke.mjs && node /tmp/pi-ai-runtime-smoke.mjs
• TypeScript checks for shared, ai, server, ui, and pi-extension via TypeScript 5.8.3
• git diff --check

Closes #789