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>

app/Livewire/Server/New/ByHetzner.php

@@ -157,7 +157,7 @@ class ByHetzner extends Component
                 'selectedHetznerSshKeyIds.*' => 'integer',
                 'enable_ipv4' => 'required|boolean',
                 'enable_ipv6' => 'required|boolean',
-                'cloud_init_script' => 'nullable|string',
+                'cloud_init_script' => ['nullable', 'string', new \App\Rules\ValidCloudInitYaml],
                 'save_cloud_init_script' => 'boolean',
                 'cloud_init_script_name' => 'nullable|string|max:255',
                 'selected_cloud_init_script_id' => 'nullable|integer|exists:cloud_init_scripts,id',