Move Test Config to Jsonc

[benrr101]

Description

As a byproduct of the PR pipeline, I got annoyed with the red squigglies under the comments in the test config.json file. Technically speaking jsonc files support comments while json does not. So, this small PR does the following:

• Changes the config.default.json file to config.default.jsonc
• Rewrites the Config.Load logic so that it tries the environment variable path first, config.jsonc second, then config.json before failing.
• Makes Config class #nullable
• Sync'd the config.default.json file with the fields that actually exist (and removed unused fields).
• Remove dependency on Newtonsoft.Json from TestUtilities and push it back to ManualTests.

🤖

Used 🤖 to find remaining references to config.json and replace with references to config.jsonc

Issues

N/A

Testing

Everything builds, but I want to validate with a PR run before taking it out of draft state.

[Copilot]

Pull request overview

Copilot reviewed 14 out of 16 changed files in this pull request and generated 8 comments.

[paulmedynski]

Looks good, modulo Copilot feedback.