Update assert_emitted to use Mapping (#1101)

Author: oliwenmandiamond

src/ophyd_async/testing/_assert.py

@@ -132,7 +132,7 @@ async def assert_describe_signal(signal: SignalR, /, **metadata):
     assert actual_datakey == expected_datakey
 
 
-def assert_emitted(docs: dict[str, list[dict]], **numbers: int):
+def assert_emitted(docs: Mapping[str, list[dict]], **numbers: int):
     """Assert emitted document generated by running a Bluesky plan.
 
     :param docs: A mapping of document type -> list of documents that have been emitted.