Startup only queues an adapterless platform when it can heal on its own (an
unregistered plugin with a bot credential), but the reconnect watcher's
`if not adapter:` branch kept every queued platform retrying forever. A
platform that became builtin-probe-failing or registered-but-None after being
queued (e.g. queued from a runtime fatal or startup connect failure) would
re-warn at the backoff cap indefinitely (#5196 fleet nodes).
Move the predicate into GatewayAdapterLifecycleMixin._adapter_may_heal and use
it at both sites: the watcher now marks such a platform fatal/needs_attention
with the non-retrying message and drops it from the queue. The late imports
of _BUILTIN_ADAPTERS / platform_registry / _platform_has_bot_credential keep
the same seams tests patch.
Tests: fold the builtin-missing-creds "fatal, not queued" case into the
missing-adapter test, run the heal test straight-line, and pin the watcher
dropping a registered plugin that still returns None. Still 2 tests.
fa12a55314