When the agent is running and the user sends multiple TEXT messages in
rapid succession, base.py's active-session branch stored the pending
event as a single-slot replacement:
self._pending_messages[session_key] = event
Three rapid messages A, B, C landed as: A (interrupts), B (replaces A
before consumer reads), C (replaces B). Only C reached the next turn —
A and B were silently dropped. This is the symptom in #4469.
Route the follow-up through merge_pending_message_event(..., merge_text=True)
so TEXT events accumulate into the existing pending event's text instead
of clobbering it. Photo and media bursts already merged through the same
helper; this just extends the merge_text path (already used by the
Telegram bursty-grace branch in gateway/run.py) to all platforms.
Test exercises BasePlatformAdapter.handle_message directly with the
session marked active and asserts three rapid TEXT events merge to
'part two\\npart three' rather than dropping the middle message.
Sanity-checked the test would fail without the fix.
Credits @devorun for the original investigation and analysis in #4491
that surfaced the underlying queue handling, though their fix targeted
GatewayRunner._pending_messages which is now dead state on main.
