Incorrect error response status code for M1_ContentHostingProvisioning_purgeContentHostingCache operation about rt-5gms-application-server

[rjb1000]

I have created a Pull Request 5G-MAG/rt-common-shared#16 to add 422 (Unprocessable Entity) to the M3 API as a response to the purgeContentHostingCache operation.

[rjb1000]

I have also raised 5G-MAG/Standards#40 to add this error response code (as well as a few others) to the purgeContentHostingCache operation in the M1 RESTful API specified in 3GPP TS 26.512.

[rjb1000]

The ProblemDetails JSON structure conveyed as the body of the HTTP error response should clearly distinguish the various error cases:

1. The request message body does not conform to the required x-www-form-urlencoded syntax.
2. The form supplied in the request message body is syntactically correct, but does not include the required key.
3. The form supplied in the request message body is syntactically correct, but includes unrecognised additional keys.
   • (Less important for the Application Server to check? Maybe it could simply ignore additional keys in the supplied form?)
4. The form supplied in the request message body is syntactically correct, but the supplied regular expression does not conform to the required syntax.
   • e.g. the regular expression parser threw an error when attempting to compile it.