genai: omit empty text parts from session history

When adding to session history, remove empty text parts.

This is the same change as googleapis/google-cloud-go#10362.

It fixes TestLive/tools/direct.

genai/chat.go

@@ -70,8 +70,20 @@ func (cs *ChatSession) addToHistory(cands []*Candidate) bool {
 			return false
 		}
 		c.Role = roleModel
-		cs.History = append(cs.History, c)
+		cs.History = append(cs.History, copySanitizedModelContent(c))
 		return true
 	}
 	return false
 }
+
+// copySanitizedModelContent creates a (shallow) copy of c with role set to
+// model and empty text parts removed.
+func copySanitizedModelContent(c *Content) *Content {
+	newc := &Content{Role: roleModel}
+	for _, part := range c.Parts {
+		if t, ok := part.(Text); !ok || len(string(t)) > 0 {
+			newc.Parts = append(newc.Parts, part)
+		}
+	}
+	return newc
+}

genai/client_test.go

@@ -30,6 +30,7 @@ import (
 	"testing"
 	"time"
 
+	"github.com/googleapis/gax-go/v2/apierror"
 	"google.golang.org/api/googleapi"
 	"google.golang.org/api/iterator"
 	"google.golang.org/api/option"
@@ -391,13 +392,17 @@ func TestLive(t *testing.T) {
 			if c := "Mountain View"; !strings.Contains(locArg, c) {
 				t.Errorf(`FunctionCall.Args["location"]: got %q, want string containing %q`, locArg, c)
 			}
-			res, err = session.SendMessage(ctx, FunctionResponse{
+			res, err = session.SendMessage(ctx, Text("response:"), FunctionResponse{
 				Name: movieTool.FunctionDeclarations[0].Name,
 				Response: map[string]any{
 					"theater": "AMC16",
 				},
 			})
 			if err != nil {
+				if ae, ok := err.(*apierror.APIError); ok {
+					t.Fatal(ae.Unwrap())
+
+				}
 				t.Fatal(err)
 			}
 			checkMatch(t, responseString(res), "AMC")