Test Proxy mismatch message is flipped

I spent more time than I care to admit confounded by this error:

`
There are differences between request and recordentry bodies:
.useManagedIdentity: Missing in record JSON
`

To me, this means that the property is present in the request, but is missing from the record. But actually, it was the other way around - the property was missing in the request, but present in the record.

Start by locating the Test Proxy code that formats the request and recordentry body mismatch message, then reproduce or inspect the missing-property case described here. Done means the message clearly identifies whether the property is missing from the request or from the record.

Written by the indexing model from the issue text.