Fix DefaultAzureCredential skipping managed identity in Azure Container Instances #23894
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Closes #23890 by fixing a regression in #23273, which had DefaultAzureCredential interpret a non-JSON response to its IMDS probe as indicating the server isn't IMDS or a supported imitator (I've filled the test gap that enabled the regression). This behavior is reasonable for IMDS, which always responds with JSON, but incorrect in general because ACI managed identity responds to the probe with a flat string. Sadly, there's no good way to identify ACI as the host, so the fix here is to check for its error message in the response 😞