Move job summary opt-out check to plugin service classes

[mw-kapilg]

Summary

• Update MW_GENERATE_JOB_SUMMARY environment variable name to MW_GENERATE_SUMMARY
• Move the MW_GENERATE_SUMMARY environment variable check from the plugin implementations to their respective service classes. When job summary is disabled, plugins are no longer instantiated rather than being instantiated and doing nothing

Changes

• TestResultsSummaryPluginService.m — guard plugin creation with MW_GENERATE_SUMMARY check
• getDefaultPlugins.m — only add build summary plugin when MW_GENERATE_SUMMARY is "true"
• TestResultsSummaryPlugin.m — removed env var check (plugin always generates artifacts when instantiated)
• BuildSummaryPlugin.m — removed env var check
• ParallelizableBuildSummaryPlugin.m — removed env var check
• testResultsSummary.unit.test.ts — updated test to expect null on invalid JSON