Phase 4: Remove MauiProgram.Container usage from builder extensions #694

Scope:\n- Update MauiAppBuilderExtensions to stop reading through MauiProgram.Container.\n- Move registration-time service resolution into proper DI factories or startup wiring.\n- Keep the existing service registrations intact.\n\nAcceptance criteria:\n- Extension methods do not rely on the global container.\n- App startup wiring still resolves all required services.