fix(client): slow networks no longer drop working connections#19426

fix(client): scope slow-link frame tracking to socket lifetime

test(connection): focus slow-link and refocus coverage

Merge remote-tracking branch 'origin/main' into t3/slow-links-keep-connections

docs(client): say why a refocus does not resubscribe

fix(client): count frames on arrival and keep the default ping interval

fix(client): tighten slow-link probe extension and keep refocus retries

fix(client): data still arriving keeps a slow health check alive

fix(mobile): slow networks no longer drop working connections