feat(otelsetup): replace ENABLE_DEBUG_LISTENER env var with DebugListener config field

Callers now set cfg.DebugListener instead of relying on an env var, so
the activation policy (e.g. tie to debug log level) lives in the caller
rather than in the library. Removes the ENABLE_DEBUG_LISTENER env var
from the public surface of InitProviders entirely.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>