ssh/tailssh: fix data race on conn auth state in OnPolicyChange

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OnPolicyChange can observe a conn in activeConns before authentication
completes. The previous `c.info == nil` guard was itself a data race
against clientAuth writing c.info, and even when c.info appeared
non-nil, c.localUser could still be nil, causing a nil pointer
dereference at c.localUser.Username.

Add an authCompleted atomic.Bool to conn, stored true after all auth
fields are written in clientAuth. OnPolicyChange checks this atomic
instead of c.info, which provides the memory barrier guaranteeing all
prior writes are visible to the concurrent reader.

Updates tailscale/corp#36268 (fixes, but we might want to cherry-pick)

Co-authored-by: Gesa Stupperich <gesa@tailscale.com>
Change-Id: I4c69843541f5f9f04add9bf431e320c65a203a39
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

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