test: add coverage for auto price reduction defensive checks

Add 7 test cases covering defensive validation code paths in
auto price reduction logic. Tests use model_construct() to bypass
normal validation and reach defensive checks.

Achieves 100% coverage for ad_model.py and config_model.py

Author: 1cu

tests/unit/test_ad_model.py

@@ -8,7 +8,7 @@
 
 import pytest
 
-from kleinanzeigen_bot.model.ad_model import MAX_DESCRIPTION_LENGTH, Ad, AdPartial, ShippingOption
+from kleinanzeigen_bot.model.ad_model import MAX_DESCRIPTION_LENGTH, Ad, AdPartial, Contact, ShippingOption
 from kleinanzeigen_bot.model.config_model import AdDefaults, AutoPriceReductionConfig
 from kleinanzeigen_bot.utils.pydantics import ContextualModel, ContextualValidationError
 
@@ -348,3 +348,145 @@ def test_ad_model_min_price_must_not_exceed_price(complete_ad_cfg:dict[str, obje
     }
     with pytest.raises(ContextualValidationError, match = "min_price must not exceed price"):
         Ad.model_validate(cfg_copy)
+
+
+@pytest.mark.unit
+def test_calculate_auto_price_with_missing_strategy() -> None:
+    """Test calculate_auto_price when strategy is None but enabled is True (defensive check)"""
+    from kleinanzeigen_bot.model.ad_model import calculate_auto_price
+    from kleinanzeigen_bot.model.config_model import AutoPriceReductionConfig
+
+    # Use model_construct to bypass validation and reach defensive lines 234-235
+    config = AutoPriceReductionConfig.model_construct(
+        enabled = True, strategy = None, amount = None, min_price = 50
+    )
+    result = calculate_auto_price(
+        base_price = 100,
+        auto_price_reduction = config,
+        target_reduction_cycle = 1
+    )
+    assert result == 100  # Should return base price when strategy is None
+
+
+@pytest.mark.unit
+def test_calculate_auto_price_with_missing_amount() -> None:
+    """Test calculate_auto_price when amount is None but enabled is True (defensive check)"""
+    from kleinanzeigen_bot.model.ad_model import calculate_auto_price
+    from kleinanzeigen_bot.model.config_model import AutoPriceReductionConfig
+
+    # Use model_construct to bypass validation and reach defensive lines 234-235
+    config = AutoPriceReductionConfig.model_construct(
+        enabled = True, strategy = "FIXED", amount = None, min_price = 50
+    )
+    result = calculate_auto_price(
+        base_price = 100,
+        auto_price_reduction = config,
+        target_reduction_cycle = 1
+    )
+    assert result == 100  # Should return base price when amount is None
+
+
+@pytest.mark.unit
+def test_calculate_auto_price_raises_when_min_price_none_and_enabled() -> None:
+    """Test that calculate_auto_price raises ValueError when min_price is None during calculation (defensive check)"""
+    from kleinanzeigen_bot.model.ad_model import calculate_auto_price
+    from kleinanzeigen_bot.model.config_model import AutoPriceReductionConfig
+
+    # Use model_construct to bypass validation and reach defensive line 237-238
+    config = AutoPriceReductionConfig.model_construct(
+        enabled = True, strategy = "FIXED", amount = 10, min_price = None
+    )
+
+    with pytest.raises(ValueError, match = "min_price must be specified when auto_price_reduction is enabled"):
+        calculate_auto_price(
+            base_price = 100,
+            auto_price_reduction = config,
+            target_reduction_cycle = 1
+        )
+
+
+@pytest.mark.unit
+def test_ad_validator_requires_price_when_enabled() -> None:
+    """Test Ad model validator requires price when auto_price_reduction is enabled (defensive check on Ad)"""
+    from kleinanzeigen_bot.model.config_model import AutoPriceReductionConfig
+
+    # Use model_construct to bypass AdPartial validation and reach the Ad._validate_auto_price_config validator
+    ad = Ad.model_construct(
+        title = "Test Ad",
+        category = "160",
+        description = "Test description",
+        price_type = "NEGOTIABLE",
+        shipping_type = "PICKUP",
+        type = "OFFER",
+        active = True,
+        sell_directly = False,
+        republication_interval = 7,
+        contact = Contact(name = "Test", zipcode = "12345"),
+        auto_price_reduction = AutoPriceReductionConfig(enabled = True, strategy = "FIXED", amount = 5, min_price = 50),
+        price = None
+    )
+
+    # Call the validator directly to reach line 283
+    with pytest.raises(ValueError, match = "price must be specified when auto_price_reduction is enabled"):
+        ad._validate_auto_price_config()
+
+
+@pytest.mark.unit
+def test_ad_validator_min_price_exceeds_price() -> None:
+    """Test Ad model validator when min_price > price (defensive check on Ad)"""
+    from kleinanzeigen_bot.model.config_model import AutoPriceReductionConfig
+
+    # Use model_construct to bypass AdPartial validation and reach the Ad._validate_auto_price_config validator
+    ad = Ad.model_construct(
+        title = "Test Ad",
+        category = "160",
+        description = "Test description",
+        price_type = "NEGOTIABLE",
+        shipping_type = "PICKUP",
+        type = "OFFER",
+        active = True,
+        sell_directly = False,
+        republication_interval = 7,
+        contact = Contact(name = "Test", zipcode = "12345"),
+        price = 50,
+        auto_price_reduction = AutoPriceReductionConfig(enabled = True, strategy = "FIXED", amount = 5, min_price = 100)
+    )
+
+    # Call the validator directly to reach line 286
+    with pytest.raises(ValueError, match = "min_price must not exceed price"):
+        ad._validate_auto_price_config()
+
+
+@pytest.mark.unit
+def test_auto_price_reduction_config_requires_amount_when_enabled() -> None:
+    """Test AutoPriceReductionConfig validator requires amount when enabled"""
+    from kleinanzeigen_bot.model.config_model import AutoPriceReductionConfig
+
+    with pytest.raises(ValueError, match = "amount must be specified when auto_price_reduction is enabled"):
+        AutoPriceReductionConfig(enabled = True, strategy = "FIXED", amount = None, min_price = 50)
+
+
+@pytest.mark.unit
+def test_ad_validator_when_min_price_is_none() -> None:
+    """Test Ad model validator when auto_price_reduction is enabled but min_price is None (covers line 284->287 branch)"""
+    from kleinanzeigen_bot.model.config_model import AutoPriceReductionConfig
+
+    # Use model_construct to bypass validation and create an Ad with enabled=True but min_price=None
+    ad = Ad.model_construct(
+        title = "Test Ad",
+        category = "160",
+        description = "Test description",
+        price_type = "NEGOTIABLE",
+        shipping_type = "PICKUP",
+        type = "OFFER",
+        active = True,
+        sell_directly = False,
+        republication_interval = 7,
+        contact = Contact(name = "Test", zipcode = "12345"),
+        price = 100,
+        auto_price_reduction = AutoPriceReductionConfig.model_construct(enabled = True, strategy = "FIXED", amount = 5, min_price = None)
+    )
+
+    # This should pass validation (line 284 condition is False, goes to line 287)
+    result = ad._validate_auto_price_config()
+    assert result == ad