fix(ext/node): gracefully exit on invalid NODE_CHANNEL_FD instead of panicking (#32421)
## Summary
- When process managers like PM2 set `NODE_CHANNEL_FD` to a file
descriptor that isn't a valid IPC pipe, Deno would panic during
bootstrap with "Bootstrap exception"
- Now it prints a clear error message and exits with code 1 instead
- Adds a spec test verifying the graceful exit behavior
Closes #29007
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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>