fix(spanner): replace multiplexed session request loop with shared in-flight creation (#14215)

Fixes: https://github.com/googleapis/google-cloud-go/issues/14212

Reworked multiplexed session initialization to use a shared in-flight
creation latch instead of the old request
goroutine and unbuffered channels. This fixes the startup deadlock
triggered by a cancelled read-only transaction
waiter and adds regression coverage for cancellation and client shutdown
while session creation is in flight.
  
sessionManager used an unbuffered multiplexedSessionReq channel and a
single goroutine to coordinate multiplexed session creation. A cancelled
waiter could cause that goroutine to return, leaving future callers
blocked forever trying to send on the request channel during
takeMultiplexed.

Replace that request/ack flow with a shared in-flight creation record on
sessionManager. When no multiplexed session exists, the manager starts
one background creation with a manager-owned context and exposes a done
channel for all waiters in that creation epoch. Waiters now block on
done or their own context, and completion is broadcast by closing the
shared channel. This removes the deadlock where one cancelled context
could kill shared coordination, and it also lets Client.Close release
any waiters blocked behind an in-flight initial create.