refactor: remove model.ExperimentOrchestraClient (#284)

* ongoing

* while there, make sure we test everything

* reorganize previous commit

* ensure we have reasonable coverage in session

The code in here would be better with unit tests. We have too many
integration tests and the tests overall are too slow. But it's also
true that I should not write a giant diff as part of this PR.

internal/engine/session_internal_test.go

@@ -208,3 +208,42 @@ func TestSessionMaybeLookupLocationContextLookupLocationContextFailure(t *testin
 		t.Fatal("not the error we expected", err)
 	}
 }
+
+func TestSessionFetchURLListWithCancelledContext(t *testing.T) {
+	sess := &Session{}
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel() // cause failure
+	resp, err := sess.FetchURLList(ctx, model.URLListConfig{})
+	if !errors.Is(err, context.Canceled) {
+		t.Fatal("not the error we expected", err)
+	}
+	if resp != nil {
+		t.Fatal("expected nil response here")
+	}
+}
+
+func TestSessionFetchTorTargetsWithCancelledContext(t *testing.T) {
+	sess := &Session{}
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel() // cause failure
+	resp, err := sess.FetchTorTargets(ctx, "IT")
+	if !errors.Is(err, context.Canceled) {
+		t.Fatal("not the error we expected", err)
+	}
+	if resp != nil {
+		t.Fatal("expected nil response here")
+	}
+}
+
+func TestSessionFetchPsiphonConfigWithCancelledContext(t *testing.T) {
+	sess := &Session{}
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel() // cause failure
+	resp, err := sess.FetchPsiphonConfig(ctx)
+	if !errors.Is(err, context.Canceled) {
+		t.Fatal("not the error we expected", err)
+	}
+	if resp != nil {
+		t.Fatal("expected nil response here")
+	}
+}