Every Telegram httpx client (DoH fallback discovery, both PTB HTTPXRequest
pools, and each lazily rebuilt TelegramFallbackTransport pool) was built with
verify=True, so httpx ran ssl.create_default_context() and a full CA load on
the gateway event loop per client, per reconnect. A slow trust store stalled
the loop for most of the connect deadline on every attempt.
agent.ssl_verify already caches one platform-trust client context
(_shared_context(None)); expose it as platform_ssl_context() and pass it as
verify= to every Telegram client and transport. Verification is unchanged
(CERT_REQUIRED + check_hostname via the truststore-injected context), proxies
still flow through httpx's proxy/env handling, and the first build is ~3 ms
because the platform verifier loads no CA file.
This is the minimal slice of #133340: the PR's single-flight Future, worker
thread and to_thread client construction are unnecessary once the context is
cached and shared.
Fixes#133339
Salvaged from #133340.
785cf8a3de