test: pin the snapshot fixture to Home Assistant's extension

The ported suite passes in CI and fails locally on the same commit, same
Python (3.14.7) and byte-identical dependency versions (syrupy 6.0.0,
pytest 9.0.3, pytest-homeassistant-custom-component 0.13.365, HA
2026.9.2): tests/test_conversation.py::test_model_args fails with
"Snapshot 'test_model_args[subentry_options0]' does not exist!" even
though tests/snapshots/test_conversation.ambr contains exactly that key.

Cause: syrupy and pytest-homeassistant-custom-component both register a
`snapshot` fixture via the pytest11 entry point, and the plugin loaded
last wins. syrupy's stores snapshots in `__snapshots__/`;
HomeAssistantSnapshotExtension overrides dirname() to `snapshots/`,
which is where core's ported .ambr files are. Entry-point iteration
order is install/filesystem dependent, so which one wins varies per
machine -- confirmed by running with --snapshot-update locally, which
wrote a new file to tests/__snapshots__/ instead of finding the
committed one under tests/snapshots/.

That means the green CI run was luck, not design: any change to install
order could have flipped it, and the failure mode is a *missing*
snapshot, which reads like a stale fixture rather than a harness bug.

Fixed the way core's own tests/conftest.py does it -- redefine
`snapshot` in conftest, since a conftest fixture always takes precedence
over a plugin's. This is the one piece of core's conftest the port
dropped.

Local: 137 passed (was 136 passed, 1 failed). Matches CI's 137.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NxJYWgGJtCw13qUHZtG8ki

tests/conftest.py

@@ -41,6 +41,8 @@ from homeassistant.helpers import llm
 from homeassistant.setup import async_setup_component
 
 from pytest_homeassistant_custom_component.common import MockConfigEntry
+from pytest_homeassistant_custom_component.syrupy import HomeAssistantSnapshotExtension
+from syrupy.assertion import SnapshotAssertion
 
 pytest_plugins = "pytest_homeassistant_custom_component"
 
@@ -51,6 +53,25 @@ def auto_enable_custom_integrations(enable_custom_integrations):
     return
 
 
+@pytest.fixture
+def snapshot(snapshot: SnapshotAssertion) -> SnapshotAssertion:
+    """Return the snapshot fixture with Home Assistant's extension.
+
+    Both pytest-homeassistant-custom-component and syrupy register a
+    `snapshot` fixture through the pytest11 entry point, and the plugin that
+    happens to load last wins. syrupy's own fixture stores snapshots in
+    `__snapshots__/`, while HomeAssistantSnapshotExtension uses `snapshots/`
+    -- which is where core's ported .ambr files live. Whichever fixture wins
+    is therefore decided by entry-point iteration order, which varies between
+    machines: CI resolved the HA one and passed, while a local run resolved
+    syrupy's and failed with "snapshot does not exist".
+
+    Redefining it here (as core's own tests/conftest.py does) pins it: a
+    conftest fixture always takes precedence over a plugin's.
+    """
+    return snapshot.use_extension(HomeAssistantSnapshotExtension)
+
+
 @pytest.fixture
 def mock_conversation_subentry_data() -> dict[str, Any]:
     """Mock subentry data."""