Separate initialization and enablement flags for OpenTelemetry disabled mode

In OpenTelemetry Python setup code, using one _initialized flag for both "setup ran" and "SDK enabled" can cause disabled mode bugs: setup returns early for OTEL_SDK_DISABLED=true, but later app instrumentation and flush paths still execute because they only check _initialized. This leads to accidental framework instrumentation and noisy shutdown warnings when proxy providers lack force_flush. Verified in local reproduction on 2026-05-15 with FastAPI + opentelemetry-instrumentation-fastapi.

Track telemetry SDK state with separate flags (e.g., initialized vs enabled), and gate app instrumentation/shutdown flush on the enabled flag when OTEL_SDK_DISABLED=true. Verify by asserting no OpenTelemetry app instrumentation marker is set and no flush warnings are logged in disabled mode.