ssh/tailssh: fix race between termination message write and session teardown

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When a recording upload fails mid-session, killProcessOnContextDone
writes the termination message to ss.Stderr() and kills the process.
Meanwhile, run() takes the ss.ctx.Done() path and proceeds to
ss.Exit(), which tears down the SSH channel. The termination message
write races with the channel teardown, so the client sometimes never
receives it.

Fix by adding an exitHandled channel that killProcessOnContextDone
closes when done. run() now waits on this channel after ctx.Done()
fires, ensuring the termination message is fully written before
the SSH channel is torn down.

Fixes #7707

Change-Id: Ib60116c928d3af46d553a4186a72963c2c731e3e
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

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