refactor: extract EngineContractMixin base class for plugin tests

- Add tests/contracts/engine_contract.py with shared Engine/Session tests
- TestKokoroEngineContract and TestSuperTonicEngineContract inherit from it
- Eliminates protocol test duplication between plugins
- Any new plugin just inherits EngineContractMixin to verify compliance

tests/contracts/engine_contract.py

@@ -0,0 +1,120 @@
+"""Base contract tests for Engine implementations.
+
+Any new TTS plugin must inherit from these classes to verify
+it satisfies the Engine/EngineSession protocol.
+
+Usage:
+    from tests.contracts.engine_contract import EngineContractMixin
+
+    class TestMyEngine(EngineContractMixin):
+        @pytest.fixture
+        def engine(self):
+            return create_my_engine()
+"""
+
+from __future__ import annotations
+
+import pytest
+
+from abogen.tts_plugin.engine import Engine, EngineSession
+from abogen.tts_plugin.errors import EngineError
+from abogen.tts_plugin.types import (
+    AudioFormat,
+    ParameterValues,
+    SynthesisRequest,
+    SynthesizedAudio,
+    VoiceSelection,
+)
+
+
+class EngineContractMixin:
+    """Base contract tests for Engine implementations.
+
+    Subclasses must define a module-level ``engine`` fixture returning
+    a fully initialized Engine instance. The tests below will use it
+    via pytest's standard fixture resolution.
+    """
+
+    def _req(self, text: str = "Hello", voice: str | None = None) -> SynthesisRequest:
+        return SynthesisRequest(
+            text=text,
+            voice=VoiceSelection(source="builtin", key=voice or "default"),
+            parameters=ParameterValues(values={}),
+            format=AudioFormat(mime="audio/wav", extension="wav"),
+        )
+
+    # ── Engine protocol ──────────────────────────────────────
+
+    def test_engine_satisfies_protocol(self, engine: Engine) -> None:
+        assert isinstance(engine, Engine)
+
+    def test_create_session_returns_session(self, engine: Engine) -> None:
+        session = engine.createSession()
+        assert isinstance(session, EngineSession)
+        session.dispose()
+
+    def test_create_session_returns_new_instances(self, engine: Engine) -> None:
+        s1 = engine.createSession()
+        s2 = engine.createSession()
+        assert s1 is not s2
+        s1.dispose()
+        s2.dispose()
+
+    def test_dispose_is_idempotent(self, engine: Engine) -> None:
+        engine.dispose()
+        engine.dispose()
+
+    def test_create_session_after_dispose_raises(self, engine: Engine) -> None:
+        engine.dispose()
+        with pytest.raises(EngineError):
+            engine.createSession()
+
+    # ── Session protocol ─────────────────────────────────────
+
+    def test_session_satisfies_protocol(self, engine: Engine) -> None:
+        session = engine.createSession()
+        assert isinstance(session, EngineSession)
+        session.dispose()
+        engine.dispose()
+
+    def test_session_synthesize_returns_audio(self, engine: Engine) -> None:
+        session = engine.createSession()
+        result = session.synthesize(self._req())
+        assert isinstance(result, SynthesizedAudio)
+        assert isinstance(result.data, bytes)
+        assert len(result.data) > 0
+        session.dispose()
+        engine.dispose()
+
+    def test_session_dispose_is_idempotent(self, engine: Engine) -> None:
+        session = engine.createSession()
+        session.dispose()
+        session.dispose()
+        engine.dispose()
+
+    def test_session_synthesize_after_dispose_raises(self, engine: Engine) -> None:
+        session = engine.createSession()
+        session.dispose()
+        with pytest.raises(EngineError):
+            session.synthesize(self._req())
+        engine.dispose()
+
+    def test_session_multiple_synthesize(self, engine: Engine) -> None:
+        session = engine.createSession()
+        r1 = session.synthesize(self._req())
+        r2 = session.synthesize(self._req())
+        assert isinstance(r1.data, bytes)
+        assert isinstance(r2.data, bytes)
+        session.dispose()
+        engine.dispose()
+
+    # ── Lifecycle ────────────────────────────────────────────
+
+    def test_full_lifecycle(self, engine: Engine) -> None:
+        s1 = engine.createSession()
+        s2 = engine.createSession()
+        s1.synthesize(self._req())
+        s2.synthesize(self._req())
+        s1.dispose()
+        s2.dispose()
+        engine.dispose()