Add validation to engine dictionary

[brokoli777]

Fixes #773

This adds a local function to see if custom_values dictionary only contains strings. It will throw an error if the dictionary is not valid.
I have also added a test case for testing the code change.

Please let me know if there are any changes to be made, as it is my first time contributing to this codebase.

[brokoli777]

Unfortunately, I couldn't run the unit tests due to the following error, but I tested it separately.

I have installed the packages from requirements.txt #Resolved

[marina-p]

Hi, thank you for adding this validation. The PR and test looks good. We've had CI checks broken for a few weeks, I just re-enabled them and everything including the new test is passing, so I'll go ahead and merge this: https://github.com/microsoft/restler-fuzzer/actions/runs/12292021287. For troubleshooting running tests locally, what was the exact command/directory/environment?

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In reply to: 2473982009

[marina-p]