feat: set peers lost when disconnected from coordinator (#11681)

Adds support to Coordination to call SetAllPeersLost() when it is closed. This ensure that when we disconnect from a Coordinator, we set all peers lost.

This covers CoderSDK (CLI client) and Agent.  Next PR will cover MultiAgent (notably, `wsproxy`).

testutil/ctx.go

@@ -22,3 +22,13 @@ func RequireRecvCtx[A any](ctx context.Context, t testing.TB, c <-chan A) (a A)
 		return a
 	}
 }
+
+func RequireSendCtx[A any](ctx context.Context, t testing.TB, c chan<- A, a A) {
+	t.Helper()
+	select {
+	case <-ctx.Done():
+		t.Fatal("timeout")
+	case c <- a:
+		// OK!
+	}
+}