Keep dev and preview servers in the foreground for Windows agents #18029

Changes

• Fixes #18019 by keeping astro dev and astro preview in the foreground when Astro detects an AI agent on Windows.


• Windows Job Objects can terminate detached descendants when the agent command exits. Agents can manage the foreground process directly, while --background remains available as an explicit opt-in. Other platforms are unchanged.

Testing

• Adds coverage for disabling agent-inferred backgrounding on Windows while preserving it on macOS and Linux.

Docs

• No docs update is needed because the existing --background interface is unchanged.