fix: validate server host configuration

[echobt]

Description

This PR adds validation to the server_host configuration option in config.rs. Previously, any string was accepted, leading to runtime failures with unclear error messages if users provided invalid hosts like URLs or IPs with ports.

Changes

• Added validation logic in set_server_host.
• Rejects empty strings.
• Rejects URLs with protocols (e.g., http://).
• Rejects hosts containing ports (e.g., 127.0.0.1:8080), instructing users to use server_port instead.
• Rejects strings with invalid characters.
• Added a unit test test_server_host_validation to verify the new validation rules.

Testing

• Added new test case test_server_host_validation which verifies that:
  • Valid hosts (localhost, 127.0.0.1) are accepted.
  • Invalid hosts (empty, URLs, IPs with ports) are rejected with appropriate error messages.
• Ran cargo test config::tests::test_server_host_validation successfully.

Closes #184