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_host_context_contract.py

@@ -0,0 +1,89 @@
+"""Contract tests for HostContext.
+
+These tests verify that HostContext satisfies the architectural requirements:
+- Minimal (3 fields maximum)
+- Frozen dataclass
+- config_dir: Path
+- logger: Logger
+- http_client: HttpClient protocol
+"""
+
+import logging
+from pathlib import Path
+
+import pytest
+
+from abogen.tts_plugin.host_context import HttpClient, HostContext
+
+
+class TestHostContextContract:
+    """Contract tests for HostContext dataclass."""
+
+    def test_is_frozen_dataclass(self) -> None:
+        assert hasattr(HostContext, "__dataclass_params__")
+        assert HostContext.__dataclass_params__.frozen is True
+
+    def test_required_fields(self, tmp_path: Path) -> None:
+        logger = logging.getLogger("test")
+
+        class FakeClient:
+            def get(self, url: str, **kwargs: object) -> object:
+                return None
+
+            def post(self, url: str, **kwargs: object) -> object:
+                return None
+
+        ctx = HostContext(
+            config_dir=tmp_path,
+            logger=logger,
+            http_client=FakeClient(),
+        )
+        assert ctx.config_dir == tmp_path
+        assert ctx.logger is logger
+
+    def test_immutability(self, tmp_path: Path) -> None:
+        class FakeClient:
+            def get(self, url: str, **kwargs: object) -> object:
+                return None
+
+            def post(self, url: str, **kwargs: object) -> object:
+                return None
+
+        ctx = HostContext(
+            config_dir=tmp_path,
+            logger=logging.getLogger("test"),
+            http_client=FakeClient(),
+        )
+        with pytest.raises(AttributeError):
+            ctx.config_dir = Path("/other")  # type: ignore[misc]
+
+    def test_max_three_fields(self) -> None:
+        """Architecture spec: HostContext is minimal (3 fields max)."""
+        import dataclasses
+
+        fields = dataclasses.fields(HostContext)
+        assert len(fields) <= 3
+
+
+class TestHttpClientProtocolContract:
+    """Contract tests for HttpClient protocol."""
+
+    def test_http_client_is_protocol(self) -> None:
+        assert hasattr(HttpClient, "__protocol_attrs__")
+
+    def test_http_client_has_get(self) -> None:
+        assert hasattr(HttpClient, "get")
+
+    def test_http_client_has_post(self) -> None:
+        assert hasattr(HttpClient, "post")
+
+    def test_http_client_satisfied(self) -> None:
+        class FakeClient:
+            def get(self, url: str, **kwargs: object) -> object:
+                return None
+
+            def post(self, url: str, **kwargs: object) -> object:
+                return None
+
+        client = FakeClient()
+        assert isinstance(client, HttpClient)