feat(07-01): extend buildZodSchema() to chain .regex() from field.validate

- Replace simple ternary assignment with let schema pattern
- Chain (schema as z.ZodString).regex() when field.validate is present
- Handle optional fields: z.string() first, then .regex() if needed, then .optional()
- VALID-01 tests all pass (GREEN): azureblob account, s3 region, gcs project_number

src/schemas/index.ts

@@ -10,9 +10,19 @@ function buildZodSchema(backendType: BackendType): z.ZodObject<Record<string, z.
   const fields = BACKEND_REGISTRY[backendType].fields;
   const shape: Record<string, z.ZodTypeAny> = {};
   for (const field of fields) {
-    shape[field.key] = field.required
+    let schema: z.ZodTypeAny = field.required
       ? z.string().min(1, `${field.label} is required`)
-      : z.string().optional();
+      : z.string();
+
+    if (field.validate) {
+      schema = (schema as z.ZodString).regex(field.validate.regex, field.validate.message);
+    }
+
+    if (!field.required) {
+      schema = (schema as z.ZodString).optional();
+    }
+
+    shape[field.key] = schema;
   }
   return z.object(shape);
 }