feat: add contract test suite for Plugin API

Create reusable contract tests for TTS Plugin Architecture:
- conftest.py: shared fixtures and stubs (FakeEngine, FakeSession, etc.)
- test_types_contract.py: value object contracts (frozen, immutability, equality)
- test_errors_contract.py: error hierarchy contracts
- test_manifest_contract.py: manifest type contracts
- test_engine_contract.py: Engine protocol contracts (lifecycle, dispose)
- test_session_contract.py: EngineSession protocol contracts
- test_capabilities_contract.py: capability protocol contracts
- test_host_context_contract.py: HostContext contracts
- test_plugin_contract.py: plugin contract (exports, create_engine)

124 tests covering all public API contracts.

tests/contracts/test_errors_contract.py

@@ -0,0 +1,85 @@
+"""Contract tests for error hierarchy.
+
+These tests verify that the error hierarchy satisfies the architectural requirements:
+- All errors inherit from EngineError
+- EngineError inherits from Exception
+- Each error type is properly classified
+"""
+
+import pytest
+
+from abogen.tts_plugin.errors import (
+    CancelledError,
+    ConfigurationError,
+    EngineError,
+    InternalError,
+    InvalidInputError,
+    ModelLoadError,
+    ModelNotFoundError,
+    NetworkError,
+)
+
+
+class TestErrorHierarchyContract:
+    """Contract tests for the error hierarchy."""
+
+    def test_engine_error_is_exception(self) -> None:
+        assert issubclass(EngineError, Exception)
+
+    def test_all_errors_inherit_from_engine_error(self) -> None:
+        error_classes = [
+            ModelNotFoundError,
+            ModelLoadError,
+            NetworkError,
+            InvalidInputError,
+            ConfigurationError,
+            CancelledError,
+            InternalError,
+        ]
+        for error_class in error_classes:
+            assert issubclass(error_class, EngineError), (
+                f"{error_class.__name__} must inherit from EngineError"
+            )
+
+    def test_all_errors_are_catchable(self) -> None:
+        error_classes = [
+            EngineError,
+            ModelNotFoundError,
+            ModelLoadError,
+            NetworkError,
+            InvalidInputError,
+            ConfigurationError,
+            CancelledError,
+            InternalError,
+        ]
+        for error_class in error_classes:
+            with pytest.raises(EngineError):
+                raise error_class("test message")
+
+    def test_error_message_preserved(self) -> None:
+        msg = "Model not found: bert-base"
+        with pytest.raises(ModelNotFoundError, match=msg):
+            raise ModelNotFoundError(msg)
+
+    def test_error_can_be_caught_as_engine_error(self) -> None:
+        with pytest.raises(EngineError):
+            raise ModelNotFoundError("test")
+
+    def test_cancelled_error_is_engine_error(self) -> None:
+        """CancelledError is a subtype of EngineError per architecture spec."""
+        assert issubclass(CancelledError, EngineError)
+
+    def test_error_hierarchy_no_cycles(self) -> None:
+        """Verify no circular inheritance."""
+        error_classes = [
+            EngineError,
+            ModelNotFoundError,
+            ModelLoadError,
+            NetworkError,
+            InvalidInputError,
+            ConfigurationError,
+            CancelledError,
+            InternalError,
+        ]
+        for cls in error_classes:
+            assert cls not in cls.__bases__