Fold the three re-arm helpers (_typing_retrigger_state, _clear_typing_retrigger,
_send_typing_quietly) into _retrigger_typing itself; the semantic change from #111886
is unchanged: schedule sendChatAction as a tracked background task instead of awaiting
it on the send path, one in-flight re-arm per chat, at most one per
typing_retrigger_min_interval_seconds (2s default, matching _keep_typing), and honour
typing_indicator: false, which previously only gated the refresh loop.
Tests: keep the two invariants that are red on origin/main — an intermediate send()
returns while sendChatAction is stalled, and a 20-chunk stream costs one sendChatAction
per chat — and drop the eight change-detector variants.
Co-authored-by: aurel282 <aurelien.gek@gmail.com>
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