net/http: count a lost HTTP/2 ping exactly once, and never on a failed connection

[ali-sayyah]

The HTTP/2 health-check timer is re-armed after every ReadFrame
return, including the final erroring one, and no close path stops it:
one SendPingTimeout after every connection close a post-mortem health
check pings the dead connection, fails immediately, and reports a
spurious CountError("conn_close_lost_ping"). A genuinely lost ping is
counted twice: the real detection, then the post-close echo.

Stop the timer when the read loop exits, publish the read loop's
terminal exit under cc.mu before the failure is counted, and make
lost-ping classification a single-claim close: the eligibility check
and the claim share one critical section in closeForLostPing, so
overlapping health checks, or a ping racing the read loop's terminal
exit, can never produce a second count. The CountError callback runs
outside the lock. A ping that fails on a connection that is live at
claim time is still counted and still closes the connection,
preserving write-blocked-ping detection.

The equivalent x/net change (golang/net#262) fixes the legacy
transport used on Go versions before 1.27 and with the http2legacy
build tag.

Fixes #80920

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