When testing Copilot Chat in automated scenarios with VSC as the Extension Host, Copilot Chat needs to use environment-variable based authentication as well as have some additional access to services that were previously blocked when running in ExtensionMode.Test.

• Required moving TestAuthenticationService out of test and into platform code. Renamed to StaticGitHubAuthenticationService in an attempt to capture its impl. Though, it really is just for Test (be it stest or heavier weight e2e eval tests).
• The token provider needs to be explicitly passed instead of defaulted due to import zone violations.
• Added an isScenarioAutomation flag. Eventually, it would be nice to move onto EnvService, but EnvService is not yet initialized in some locations that value of the flag is needed.