Ensure no DurableOrchestrationClient methods fail to decode mesages

When using aiohttp, the library will, by default, validate that the encoding of a server message matches its format but, with Azure Functions and many other services, the message encoding metadata is often not up to data. This mismatch yields a `mimetype encoding` runtime exception that we've already had to fix in multiple places.

We should create a robust tester to ensure these kinds of error no longer take place in our codebase.

Start at the DurableOrchestrationClient methods and their aiohttp response handling, then inspect the existing test suite for coverage of message decoding. Reproduce the mimetype-encoding failure and add coverage showing that each relevant method handles mismatched server encoding metadata without raising the runtime exception.

Written by the indexing model from the issue text.