fetch: let an explicit timeout extend the socket idle deadline - #33647

A `{timeout: false}` request coalesced onto an h2 session can now be killed by a sibling's short explicit `timeout`: `effective_idle_timeout_seconds()` returns 0 for `disable_timeout`, so it contributes nothing to the `max()` and a `{timeout: 5000}` sibling arms the shared socket at 5s — when it fires, `on_long_timeout` → `session.on_close` fails **every** stream with no per-stream `disable_timeout` re-check. Before this PR the same session armed the global 300s default, so the `{timeout: false}