chore(context engine): Keep 1 kill switch option for all of context engine indexing

[Mihir-Mavalankar]

PR Details

• Renames explorer.service_map.enable to explorer.context_engine_indexing.enable and moves the kill switch from build_service_map to schedule_context_engine_indexing_tasks, so disabling it prevents all context engine indexing tasks (both project knowledge indexing and service map building) from being dispatched.
• Updates tests accordingly.

[cursor]

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