Move the crash handler process spawn to a background thread

[Anthony-Eid]

Moves the crash handler subprocess spawn off the main thread to speed up startup, especially on Windows where process creation is slower.

Previously crashes::init did part of its work synchronously when it was called, including spawn_crash_handler, which launches the zed --crash-handler child process (a synchronous CreateProcessW on Windows). Because init is evaluated inline as the argument to background_executor().spawn(crashes::init(...)) in main.rs, that subprocess spawn ran on the main thread during startup rather than on the executor.

This PR makes connect_and_keepalive a fully async fn, so all of that work, including the subprocess spawn, now runs on the background executor instead of blocking the main thread.

I don't have a Windows machine to capture before/after numbers, but the crash handler spawn is clearly on the startup critical path. Logs in the Windows slow startup reports show it taking ~100–700ms between spawning crash handler process and connected to crash handler process (e.g. #40621, #54856), all of which previously blocked the main thread.

Related to #49442

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content adheres to Zed's UI standards (UX/UI and icon guidelines)
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Release Notes:

• Improved startup performance