Refactored subject selection builder to reduce cognitive complexity

- Extracted `_preprocess_criteria()` and `_dispatch_criteria_key()` from `_add_variable_selection_criteria`
- Removed duplicated preprocessing logic after extraction
- Added descriptive docstrings for both new helper methods
- Wrote pytest coverage for criteria dispatch and preprocessing paths
- Brought method below SonarQube cognitive complexity threshold

Author: Andyg79

tests_utils/test_subject_selection_query_builder.py

@@ -0,0 +1,73 @@
+import pytest
+from utils.oracle.subject_selection_query_builder import (
+    SubjectSelectionQueryBuilder,
+    SubjectSelectionCriteriaKey,
+)
+from classes.subject import Subject
+from classes.user import User
+
+
+@pytest.fixture
+def builder():
+    return SubjectSelectionQueryBuilder()
+
+
+@pytest.fixture
+def dummy_subject():
+    subject = Subject()
+    subject.screening_status_change_date = None
+    subject.date_of_death = None
+    return subject
+
+
+@pytest.fixture
+def dummy_user():
+    return User()
+
+
+def test_add_criteria_subject_age_y_d(builder, dummy_user, dummy_subject):
+    # This format triggers the 'y/d' logic branch
+    criteria = {"subject age (y/d)": "60/0"}
+
+    builder._add_variable_selection_criteria(criteria, dummy_user, dummy_subject)
+    where_clause = " ".join(builder.sql_where)
+
+    assert "c.date_of_birth" in where_clause
+    assert "ADD_MONTHS(TRUNC(TRUNC(SYSDATE))" in where_clause
+
+
+def test_add_criteria_subject_hub_code_with_enum(builder, dummy_user, dummy_subject):
+    criteria = {"subject hub code": "user organisation"}
+    builder._add_variable_selection_criteria(criteria, dummy_user, dummy_subject)
+    sql = " ".join(builder.sql_where)
+    assert "c.hub_id" in sql
+    assert "SELECT hub.org_id" in sql
+
+
+def test_invalid_criteria_key_raises_exception(builder, dummy_user, dummy_subject):
+    criteria = {"invalid key": "value"}
+    with pytest.raises(Exception):
+        builder._add_variable_selection_criteria(criteria, dummy_user, dummy_subject)
+
+
+def test_preprocess_commented_criterion_skips_processing(builder, dummy_subject):
+    result = builder._preprocess_criteria(
+        "subject age", "#this is ignored", dummy_subject
+    )
+    assert result is False
+
+
+def test_dispatch_known_key_executes(builder, dummy_user, dummy_subject):
+    # Arrange
+    builder.criteria_key = SubjectSelectionCriteriaKey.SUBJECT_AGE
+    builder.criteria_value = "60"
+    builder.criteria_comparator = " = "
+    builder.criteria_key_name = "subject age"
+
+    # Act
+    builder._dispatch_criteria_key(dummy_user, dummy_subject)
+
+    # Assert
+    where_clause = " ".join(builder.sql_where)
+    assert "c.date_of_birth" in where_clause
+    assert "FLOOR(MONTHS_BETWEEN(TRUNC(SYSDATE), c.date_of_birth)/12)" in where_clause