First-time users have is_tracing_enabled_in_context() = False by design
(it's a prerequisite for should_auto_collect_first_time_traces). This
caused _initialize_backend_batch to return early without creating the
batch, and _send_events_to_backend to send to a non-existent batch.
Add skip_context_check parameter to _initialize_backend_batch so the
first-time handler can bypass the guard during deferred init. Gate
backend_initialized on trace_batch_id being set. Call
_finalize_backend_batch directly instead of finalize_batch (which has
the same context guard). Sync is_current_batch_ephemeral on success
to prevent endpoint mismatch between batch creation and event send.
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