Motivation:

#2226 made the connection-permit acquire non-blocking on the event loop.
CombinedConnectionSemaphore's non-blocking path takes the global permit first
and then the per-host permit, releasing the global one if the per-host permit is
unavailable. That releaseGlobal branch is the single place the non-blocking path
can leak the global permit, yet it had no coverage: the existing
combinedNonBlockingFailsFastWhenExhausted uses equal global and per-host limits
(1, 1), so the acquire is rejected at the global gate and never reaches the
per-host rejection where releaseGlobal runs.

Modification:

Add combinedNonBlockingReleasesGlobalPermitWhenPerHostExhausted using a wider
global limit (2) than per-host (1). The non-blocking acquire passes the global
gate, is rejected by the per-host limit (asserted as
TooManyConnectionsPerHostException), and a follow-up non-blocking acquire for a
different host must succeed; proving the global permit taken during the failed
attempt was released rather than leaked.

Result:

The global-permit release path of the non-blocking combined acquire is now
covered; a regression that leaked the global permit on per-host rejection would
starve other hosts of the global permit and fail this test.