fix(vm): populate an empty guest /etc/hosts with loopback names

[shiju-nv]

Summary

A MicroVM sandbox started from an OCI image that ships an empty /etc/hosts keeps that empty file, so localhost does not resolve inside the guest. The guest init now writes the loopback names when the file is missing or empty. It leaves a non-empty file and a symlink alone.

The change covers empty or missing files only. An image that ships a non-empty hosts file with no localhost line, or only comments, is still left as the author wrote it, so localhost can still fail to resolve for those images.

Related Issue

Closes #3934.

Changes

• crates/openshell-driver-vm/scripts/openshell-vm-sandbox-init.sh: add configure_hosts, called right after configure_hostname in run_post_overlay_setup. It returns without writing when /etc/hosts is a symlink (logging a warning) or is non-empty. Otherwise it writes 127.0.0.1 localhost and ::1 localhost ip6-localhost ip6-loopback. A write failure logs a warning, lets the shell's own error reach the console, and boot continues, because missing loopback names degrade the workload but do not weaken isolation.
• crates/openshell-driver-vm/src/rootfs.rs: two tests. guest_init_populates_missing_or_empty_hosts_and_keeps_image_hosts cuts the real configure_hosts out of the shipped script and runs it under bash against a temporary root for a missing file, an empty file, an image-authored file, and a symlink to a file outside the root. guest_init_boot_path_configures_hosts_after_the_account_and_before_networking checks that run_post_overlay_setup itself calls configure_hosts after reconcile_sandbox_account and before the network step.

Testing

• Checks appropriate to the affected code and behavior pass locally: focused checks and the repo's pre-commit hook pass
• Unit tests added/updated (if applicable)
• E2E tests added/updated (if applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)