BG_TASK_COMPLETE_EVENTS_SEEN gained a session_id -> set[process_id] entry the
first time a bg task completed for a session and was never deleted anywhere, so
it grew unbounded for the server lifetime.
The entry is created in _process_one for EVERY completion, whether or not any
SSE channel/tab exists, so pruning it only when the SessionChannel is reaped
would miss the dominant headless case (task fires, tab closed or never opened —
no channel to collect). Instead the reaper now sweeps the map by DELIVERY: once
a completion is drained (its session_id removed from PENDING_BG_TASK_COMPLETIONS)
the short _move_to_finished dedup window is closed and the entry is swept, every
tick, regardless of any channel. The registry's per-process_id
_completion_consumed gate remains the primary idempotency backstop, so sweeping a
delivered session's set can never resurrect an already-delivered completion.
Session deletion also prunes the entry (new forget_bg_task_completion_dedup),
covering a session deleted while a completion is still pending (which the
delivery-gated sweep deliberately retains).
Refs #4633
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
