fix(docs): add language tag to bare code fences in testing.md

pymdownx.highlight requires language specification on code fences.
Bare ``` causes NoneType error during MkDocs build.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs-src/development/testing.md

@@ -19,7 +19,7 @@
 
 **Speicherort:** Im gleichen Verzeichnis wie der Code
 
-```
+```text
 internal/
 ├── services/
 │   ├── auth_service.go
@@ -93,7 +93,7 @@ func TestValidateEmail(t *testing.T) {
 
 **Speicherort:** `/backend/tests/`
 
-```
+```text
 backend/
 ├── consent_client.py
 ├── gdpr_api.py