Refactor: enforce canonical messages/output YAML, normalize content_file, and expand fixture-based validation tests

internal/promptdef/testdata/json_schema_without_schema_path.yaml

@@ -0,0 +1,9 @@
+id: json-schema-without-schema-path
+version: "1.0.0"
+messages:
+  - role: user
+    content: "Return JSON"
+output:
+  format: json
+  validation_mode: json_schema
+  repair_attempts: 0