When the agent is busy and the user sends multiple text follow-ups, the
interrupt-mode and steer-fallback path stored them via
merge_pending_message_event(merge_text=True), which newline-joins
consecutive TEXT messages into a SINGLE pending turn — collapsing two
separate user messages into one mashed-together turn and destroying the
message boundaries the user sees (#43066 sub-bug 2).
Route that storage through _queue_or_replace_pending_event (the same FIFO
infrastructure used by busy queue-mode and /queue) so each follow-up gets
its own next-turn slot in arrival order, while still preserving
photo-burst / album merge semantics for media. Pure queue-mode already
used FIFO; this brings the interrupt/steer-fallback path in line.
The sibling defect in #43066 (assistant messages lost after compaction)
was already fixed on main by the identity-tracking flush rewrite (#46053)
plus the pre-rotation flush (#47202), so this only addresses the
remaining busy-message-merge half.
Co-authored-by: KiruyaMomochi <65301509+KiruyaMomochi@users.noreply.github.com>
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