fix: return InvalidConfig error instead of panicking for invalid directories

[hashemix]

📌 Summary

Refactors directory validation in FileSystemService::try_new to return a proper InvalidConfig error instead of panicking when an allowed directory is invalid.

🔍 Related Issues

• Docker MCP Gateway: paths config not passed as command arguments #71

✨ Changes Made

• Add InvalidConfig(String) variant to ServiceError enum
• Replace .map_while() with .map() and proper error handling
• Convert panic to ServiceError::InvalidConfig with descriptive message

🛠️ Testing Steps

Test with an invalid directory path in the server configuration and verify a proper error is returned instead of a panic:

cargo run /invalid-dir