OPENAI_KEY as env variable and extra tests

[vpapanasta]

In this change, I am setting secrets.OPENAI_KEY as an environment variable that we can use in tests.
Also, I am adding some tests to increase code coverage in openAIChat. One test of these uses the secret OPENAI_KEY, and it was added to check that we can use the key.

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[debymf]

OPENAI_API_KEY to match the env variable that we expect?

We might have to remove this part from the test:

function saveEnvVar(testCase)
           % Ensures key is not in environment variable for tests
           openAIEnvVar = "OPENAI_API_KEY";
           if isenv(openAIEnvVar)
               key = getenv(openAIEnvVar);
               unsetenv(openAIEnvVar);
               testCase.addTeardown(@(x) setenv(openAIEnvVar, x), key);
           end
       end

I have added that because when I was running local tests on some behaviors related to checking the key it would turn out that the key was already in my path. We should just move that to where it's actually needed.

[vpapanasta]

For local debugging we should still use the manually defined OPENAI_API_KEY. I do not think it harms to keep this check to make sure that OPENAI_API_KEY is not in environment variable for tests. OPENAI_KEY defined in .github/workflows/ci.yml is a different environment variable and it is secret.