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/Rules/ValidCloudInitYaml.php

@@ -0,0 +1,55 @@
+<?php
+
+namespace App\Rules;
+
+use Closure;
+use Illuminate\Contracts\Validation\ValidationRule;
+use Symfony\Component\Yaml\Exception\ParseException;
+use Symfony\Component\Yaml\Yaml;
+
+class ValidCloudInitYaml implements ValidationRule
+{
+    /**
+     * Run the validation rule.
+     *
+     * Validates that the cloud-init script is either:
+     * - Valid YAML format (for cloud-config)
+     * - Valid bash script (starting with #!)
+     * - Empty/null (optional field)
+     */
+    public function validate(string $attribute, mixed $value, Closure $fail): void
+    {
+        if (empty($value)) {
+            return;
+        }
+
+        $script = trim($value);
+
+        // If it's a bash script (starts with shebang), skip YAML validation
+        if (str_starts_with($script, '#!')) {
+            return;
+        }
+
+        // If it's a cloud-config file (starts with #cloud-config), validate YAML
+        if (str_starts_with($script, '#cloud-config')) {
+            // Remove the #cloud-config header and validate the rest as YAML
+            $yamlContent = preg_replace('/^#cloud-config\s*/m', '', $script, 1);
+
+            try {
+                Yaml::parse($yamlContent);
+            } catch (ParseException $e) {
+                $fail('The :attribute must be valid YAML format. Error: '.$e->getMessage());
+            }
+
+            return;
+        }
+
+        // If it doesn't start with #! or #cloud-config, try to parse as YAML
+        // (some users might omit the #cloud-config header)
+        try {
+            Yaml::parse($script);
+        } catch (ParseException $e) {
+            $fail('The :attribute must be either a valid bash script (starting with #!) or valid cloud-config YAML. YAML parse error: '.$e->getMessage());
+        }
+    }
+}