Replace the runner-introspection trick in #32998 with an explicit
`set_topic_recovery_fn` setter on `BasePlatformAdapter`. The gateway
runner installs it once at adapter init; the adapter calls
`_apply_topic_recovery(event)` before any session keying.
Also apply the hook in `BasePlatformAdapter.handle_message` so the
running-agent guard and pending-message queue key off the recovered
thread_id too — not just the text-batch coalescence.
Net change vs #32998 alone: -2 files of indirection (no
`_message_handler.__self__` peek, no separate `_normalize_text_batch_source`),
+1 generic mechanism (other adapters can install their own hook later).
100536134c