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

internal/promptdef/testdata/missing_content_file.yaml

@@ -0,0 +1,9 @@
+id: missing-content-file
+version: "1.0.0"
+messages:
+  - role: user
+    content_file: ./messages/does_not_exist.tmpl
+output:
+  format: text
+  validation_mode: none
+  repair_attempts: 0