feat(wrapper): add UnwrapConn to remaining TCP conn wrappers

Add UnwrapConn() to the admission, proxyproto, conn limiter, metrics,
and observer/stats TCP connection wrappers, complementing the traffic
limiter's limitConn added in the previous commit.

This makes the unwrapConn() helper in handler/sshd able to peel through
any combination of these wrappers uniformly, keeping the pattern
complete and future-proof for handlers that assert on concrete
connection types.

internal/net/proxyproto/conn.go

@@ -14,6 +14,12 @@ type serverConn struct {
 	ctx context.Context
 }
 
+// UnwrapConn returns the underlying connection, allowing type assertions
+// through wrapper layers.
+func (c *serverConn) UnwrapConn() net.Conn {
+	return c.Conn
+}
+
 func (c *serverConn) Context() context.Context {
 	return c.ctx
 }