Follow-up hardening for the salvaged #69240 readiness gate (#67498):
- _start_polling_once now returns its (generation, progress_event) pair
so the strict cold-start gate binds to exactly the generation it
started, instead of re-reading self._polling_progress_event which a
concurrent recovery task may have replaced with a newer generation's
event (the G1/G2 race flagged in the #69240 review).
- Strict cold start no longer schedules background polling recovery: a
polling error during the readiness wait is captured by a strict
callback and fails the connect attempt immediately with a loud
OSError, so GatewayRunner disposes the partial adapter and retries
with a fresh one — no more waiting out the full readiness deadline on
a generation that already errored, and no G2-on-partial-app healing.
- After readiness is proven the strict callback delegates every later
polling error to the real background-recovery callback, preserving
the existing degraded/reconnect semantics for the polling lifetime.
- The readiness-timeout error message now states the deadline and that
the gateway will retry with a fresh adapter (loud failure, not a
silent wait).
- Regression tests: current-generation progress connects; a polling
error during strict cold start fails fast without scheduling
background recovery (the #67498 idle-threads shape); stale-generation
progress is rejected.
Progresses #67498
