ISS-320: Handle missing variable metadata in test endpoint.

This PR updates error handling in the test endpoint to return a better error message when variable metadata is missing

Steps to test:

Setup your local environment to rune the function app following the steps outlined here: https://learn.microsoft.com/en-us/azure/azure-functions/functions-run-local?tabs=v4%2Cwindows%2Ccsharp%2Cportal%2Cbash Additionally, create a local.settings.json file in the project root with the following json:
```
{
"IsEncrypted": false,
"Values": {
"FUNCTIONS_WORKER_RUNTIME": "python",
"AzureWebJobs.HttpExample.Disabled": "true"
},
"Host": {
"LocalHttpPort": 7071,
"CORS": "*",
"CORSCredentials": false
}
}
```
Run the test endpoint locally by running: func start from the root directory
Make a request to the local test function: http://localhost:7071/api/TestRule with the following data: SEND2.json(1).txt
Verify that the response correctly identifies that the user is missing variable metadata

cdisc-org / cdisc-rules-engine