🧪 Add HeadlessRunner argument parsing tests

[mleem97]

🎯 What: This PR adds unit tests for HeadlessRunner.TryHandle, focusing on argument parsing, help commands, and specific error conditions without interacting directly with external dependencies like SteamWorkshopService.
📊 Coverage: Tests the --help flag (exit code 0), invalid/unhandled arguments (returning false), and the error case for --mode publish or --upload when the --path argument is missing (exit code 2).
✨ Result: Enhanced test coverage ensures command-line validation and early-exit paths are robust, catching future regressions.

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PR created automatically by Jules for task 9389622331138274382 started by @mleem97

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