fix(evidence): use @model_validator(mode='before') so evidences appear in API responses

FastAPI 0.136.1 + Pydantic 2.13.4 serialises responses via TypeAdapter which
calls the compiled Rust validator directly, bypassing any Python-level
`model_validate` classmethod override. The @model_validator(mode='before')
decorator IS invoked by the Rust pipeline, so the evidence red/blue split and
technique field population now run on every serialisation path.

Also eager-load technique in get_test_detail to avoid lazy-load surprises.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/services/test_crud_service.py

@@ -137,13 +137,13 @@ def create_test_from_template(
 
 
 def get_test_detail(db: Session, test_id: uuid.UUID) -> Test:
-    """Fetch a test with evidences eager-loaded.
+    """Fetch a test with evidences and technique eager-loaded.
 
     Raises EntityNotFoundError if the test does not exist.
     """
     test = (
         db.query(Test)
-        .options(joinedload(Test.evidences))
+        .options(joinedload(Test.evidences), joinedload(Test.technique))
         .filter(Test.id == test_id)
         .first()
     )