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/conftest.py

@@ -0,0 +1,231 @@
+"""Shared fixtures and stubs for contract tests.
+
+This module provides minimal stub implementations that satisfy the public API
+for testing purposes. These stubs do NOT contain real business logic.
+"""
+
+from __future__ import annotations
+
+import logging
+from pathlib import Path
+from typing import Iterator
+
+import pytest
+
+from abogen.tts_plugin.engine import Engine, EngineSession
+from abogen.tts_plugin.host_context import HostContext
+from abogen.tts_plugin.types import (
+    AudioFormat,
+    Duration,
+    EngineConfig,
+    ParameterValues,
+    SynthesisRequest,
+    SynthesizedAudio,
+    VoiceSelection,
+)
+
+
+class FakeHttpClient:
+    """Stub HTTP client that satisfies the HttpClient protocol."""
+
+    def get(self, url: str, **kwargs: object) -> object:
+        return None
+
+    def post(self, url: str, **kwargs: object) -> object:
+        return None
+
+
+class FakeEngineSession:
+    """Stub EngineSession for testing protocol compliance."""
+
+    def __init__(self) -> None:
+        self._disposed = False
+
+    def synthesize(self, request: SynthesisRequest) -> SynthesizedAudio:
+        if self._disposed:
+            from abogen.tts_plugin.errors import EngineError
+
+            raise EngineError("Session disposed")
+        return SynthesizedAudio(
+            data=b"\x00" * 100,
+            format=AudioFormat(mime="audio/wav", extension="wav"),
+            duration=Duration(seconds=1.0),
+        )
+
+    def dispose(self) -> None:
+        self._disposed = True
+
+
+class FakeStreamingSession:
+    """Stub EngineSession with StreamingSynthesizer capability."""
+
+    def __init__(self) -> None:
+        self._disposed = False
+
+    def synthesize(self, request: SynthesisRequest) -> SynthesizedAudio:
+        if self._disposed:
+            from abogen.tts_plugin.errors import EngineError
+
+            raise EngineError("Session disposed")
+        return SynthesizedAudio(
+            data=b"\x00" * 100,
+            format=AudioFormat(mime="audio/wav", extension="wav"),
+            duration=Duration(seconds=1.0),
+        )
+
+    def synthesizeStream(self, request: SynthesisRequest) -> Iterator[bytes]:
+        if self._disposed:
+            from abogen.tts_plugin.errors import EngineError
+
+            raise EngineError("Session disposed")
+        for i in range(3):
+            yield b"\x00" * 50
+
+    def dispose(self) -> None:
+        self._disposed = True
+
+
+class FakeCancelableSession:
+    """Stub EngineSession with CancelableSession capability."""
+
+    def __init__(self) -> None:
+        self._disposed = False
+        self._cancelled = False
+
+    def synthesize(self, request: SynthesisRequest) -> SynthesizedAudio:
+        if self._disposed:
+            from abogen.tts_plugin.errors import EngineError
+
+            raise EngineError("Session disposed")
+        if self._cancelled:
+            from abogen.tts_plugin.errors import CancelledError
+
+            raise CancelledError("Cancelled")
+        return SynthesizedAudio(
+            data=b"\x00" * 100,
+            format=AudioFormat(mime="audio/wav", extension="wav"),
+            duration=Duration(seconds=1.0),
+        )
+
+    def cancel(self) -> None:
+        if self._disposed:
+            from abogen.tts_plugin.errors import EngineError
+
+            raise EngineError("Session disposed")
+        self._cancelled = True
+
+    def dispose(self) -> None:
+        self._disposed = True
+
+
+class FakeEngine:
+    """Stub Engine for testing protocol compliance."""
+
+    def __init__(self, session_class: type = FakeEngineSession) -> None:
+        self._disposed = False
+        self._session_class = session_class
+
+    def createSession(self) -> EngineSession:
+        if self._disposed:
+            from abogen.tts_plugin.errors import EngineError
+
+            raise EngineError("Engine disposed")
+        return self._session_class()
+
+    def dispose(self) -> None:
+        self._disposed = True
+
+
+class FakeVoiceListerEngine:
+    """Stub Engine that also implements VoiceLister."""
+
+    def __init__(self) -> None:
+        self._disposed = False
+
+    def createSession(self) -> EngineSession:
+        if self._disposed:
+            from abogen.tts_plugin.errors import EngineError
+
+            raise EngineError("Engine disposed")
+        return FakeEngineSession()
+
+    def listVoices(self, sourceId: str) -> list:
+        from abogen.tts_plugin.manifest import VoiceManifest
+
+        return [
+            VoiceManifest(id="voice1", name="Voice 1", tags=("en",)),
+            VoiceManifest(id="voice2", name="Voice 2", tags=("es",)),
+        ]
+
+    def dispose(self) -> None:
+        self._disposed = True
+
+
+class FakePreviewEngine:
+    """Stub Engine that also implements PreviewGenerator."""
+
+    def __init__(self) -> None:
+        self._disposed = False
+
+    def createSession(self) -> EngineSession:
+        if self._disposed:
+            from abogen.tts_plugin.errors import EngineError
+
+            raise EngineError("Engine disposed")
+        return FakeEngineSession()
+
+    def generatePreview(self, voice: VoiceSelection, text: str) -> SynthesizedAudio:
+        return SynthesizedAudio(
+            data=b"\x00" * 50,
+            format=AudioFormat(mime="audio/wav", extension="wav"),
+            duration=Duration(seconds=0.5),
+        )
+
+    def dispose(self) -> None:
+        self._disposed = True
+
+
+@pytest.fixture
+def fake_http_client() -> FakeHttpClient:
+    return FakeHttpClient()
+
+
+@pytest.fixture
+def host_context(tmp_path: Path, fake_http_client: FakeHttpClient) -> HostContext:
+    return HostContext(
+        config_dir=tmp_path,
+        logger=logging.getLogger("test"),
+        http_client=fake_http_client,
+    )
+
+
+@pytest.fixture
+def fake_engine() -> FakeEngine:
+    return FakeEngine()
+
+
+@pytest.fixture
+def fake_session() -> FakeEngineSession:
+    return FakeEngineSession()
+
+
+@pytest.fixture
+def default_voice() -> VoiceSelection:
+    return VoiceSelection(source="builtin", key="af_nova")
+
+
+@pytest.fixture
+def default_format() -> AudioFormat:
+    return AudioFormat(mime="audio/wav", extension="wav")
+
+
+@pytest.fixture
+def default_request(
+    default_voice: VoiceSelection, default_format: AudioFormat
+) -> SynthesisRequest:
+    return SynthesisRequest(
+        text="Hello, world!",
+        voice=default_voice,
+        parameters=ParameterValues(values={}),
+        format=default_format,
+    )