test: cover generation engine defaults

Verify omitted engines remain unset while explicit values are validated.

backend/tests/test_generation_engine_resolution.py

@@ -0,0 +1,27 @@
+"""Tests for generation request engine selection."""
+
+import pytest
+from pydantic import ValidationError
+
+from backend import models
+
+
+def _request(**kwargs) -> models.GenerationRequest:
+    return models.GenerationRequest(profile_id="profile-1", text="hello", **kwargs)
+
+
+def test_omitted_engine_does_not_override_profile_default():
+    request = _request()
+
+    assert request.engine is None
+
+
+def test_explicit_engine_is_preserved():
+    request = _request(engine="chatterbox")
+
+    assert request.engine == "chatterbox"
+
+
+def test_invalid_explicit_engine_is_rejected():
+    with pytest.raises(ValidationError):
+        _request(engine="invalid")