test: add maintenance mode test suite

[goravel-coder]

Summary

• Adds maintenance mode configuration (APP_MAINTENANCE_DRIVER, APP_MAINTENANCE_STORE) to config/app.go and env files
• Adds comprehensive test suite covering artisan down/up commands, HTTP middleware responses, file persistence, cache driver, and secret bypass

Closes goravel/docs#192

Why

This adds the maintenance mode configuration block and a full test suite to the example app, demonstrating the down/up artisan commands and the CheckForMaintenanceMode middleware that ships with goravel/gin and goravel/fiber. The test suite covers all major behaviors documented in the v1.18 configuration docs: reason text, custom status codes, redirect, view rendering, secret bypass, generated secrets, file persistence, and cache driver support.

// config/app.go
"maintenance": map[string]any{
    "driver": config.Env("APP_MAINTENANCE_DRIVER", "file"),
    "store":  config.Env("APP_MAINTENANCE_STORE", ""),
},

# Put app into maintenance mode
./artisan down --reason="Upgrading database" --status=503

# Allow bypass with a secret
./artisan down --secret=let-me-in

# Bring app back online
./artisan up