Support Unix socket connections for MySQL/MariaDB

[Copilot]

The MySQL connection string builder always wrapped the host in tcp(...), making it impossible to connect via Unix socket as supported by the Go MySQL driver.

Changes

• util/config.go: Added mysqlHasNetworkPrefix helper that detects known MySQL driver protocol prefixes (tcp, tcp4, tcp6, unix, pipe). GetConnectionString now passes the host as-is when a prefix is already present, otherwise falls back to the existing tcp(...) wrapping.
• util/config_test.go: Added TestMySQLGetConnectionString covering both TCP and Unix socket host formats.

Usage

In semaphore.json, specify the socket path using the Go MySQL driver's native syntax:

{
  "mysql": {
    "host": "unix(/var/run/mysqld/mysqld.sock)",
    "user": "semaphore",
    "pass": "password",
    "name": "semaphore"
  }
}