feat: add YAML validation for cloud-init scripts

Add ValidCloudInitYaml validation rule to ensure cloud-init scripts
are properly formatted before saving. The validator supports:
- Cloud-config YAML (with or without #cloud-config header)
- Bash scripts (starting with #!)
- Empty/null values (optional field)

Uses Symfony YAML parser to validate YAML syntax and provides
detailed error messages when validation fails.

Added comprehensive unit tests covering:
- Valid cloud-config with/without header
- Valid bash scripts
- Invalid YAML syntax detection
- Complex multi-section cloud-config

Applied validation to:
- ByHetzner component (server creation)
- CloudInitScriptForm component (script management)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

resources/views/livewire/security/cloud-init-script-form.blade.php

@@ -2,7 +2,7 @@
     <x-forms.input id="name" label="Script Name" helper="A descriptive name for this cloud-init script." required />
 
     <x-forms.textarea id="script" label="Script Content" rows="12"
-        helper="Enter your cloud-init script. Supports both bash scripts and cloud-config YAML format." required />
+        helper="Enter your cloud-init script. Supports cloud-config YAML format." required />
 
     <div class="flex justify-end gap-2">
         @if ($modal_mode)