fix(filesystem): gracefully handle unavailable directories

[eran3d]

Summary

Instead of crashing when a configured directory is unavailable (e.g., unmounted external drive, disconnected network share), the server now skips unavailable directories with a warning and continues operating with the remaining available directories.

Problem

Currently, if any directory in allowed_directories is unavailable at startup, the entire server crashes with process.exit(1). This makes ALL filesystem access unavailable, even for paths that are accessible.

This is particularly problematic for:

• Laptop users with external drives that aren't always connected
• Users with network shares that may be temporarily unavailable
• Anyone who wants to configure multiple directories where some might not always be mounted

Solution

The server now:

• Validates each directory individually
• Skips unavailable directories with a warning log message
• Continues operating with the remaining available directories
• Only exits if ALL specified directories are unavailable

Example log output

Warning: Skipping unavailable directory /Volumes/External-Drive: ENOENT: no such file or directory
Skipped 1 unavailable directory: /Volumes/External-Drive
Server starting with 2 available directories: /Users/username/Documents, /Users/username/Desktop

Testing

Tested locally with a mix of available and unavailable directories. The server now starts successfully and operates on the available directories while logging warnings about the unavailable ones.

Fixes #2815