Eight adapters (discord, telegram, wecom, matrix, whatsapp, simplex, feishu, weixin) each kept a
copy of the delayed text-batch flush that base._enqueue_text_event schedules. Two correctness
fixes had landed in single copies only: Discord's asyncio.shield around the dispatch (#12444 —
a late chunk cancelling the flush task aborted the in-flight agent turn) and WeCom/Weixin's
synchronous task-identity check before the pop (a superseded task waking late popped the event
and the successor found nothing). The other adapters carried both bugs latent.
The base now owns `_flush_text_batch` with both fixes, plus the `_pending_text_batches` /
`_pending_text_batch_tasks` dicts and `_SPLIT_THRESHOLD` / delay attrs (defaults; adapters set
their own). Platform policy goes through three small hooks instead of a copied body:
`_text_batch_delay_for(pending)` (Telegram's fast/short tiers, WeCom's attachment-only wait),
`_pop_text_batch(key)` (Feishu's side count table) and `_dispatch_text_batch(event)` (Feishu's
per-chat lock). Telegram keeps its `_flush_buffered` body because its contract differs on
purpose — a cancel after the pop must hold-and-re-raise so teardown can stop a flush — and gains
the identity check there. Matrix's `_split_threshold` is renamed to the shared `_SPLIT_THRESHOLD`;
SimpleX exposes its single delay through the shared attr names.
`helpers.TextBatchAggregator` (zero users) sits inside the revert-scheduled PLUGIN-COMPAT block
and is left for that revert.
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