[bugfix] Prevent multiple mentions being created for one mention (#4127)

# Description

> If this is a code change, please include a summary of what you've coded, and link to the issue(s) it closes/implements.
>
> If this is a documentation change, please briefly describe what you've changed and why.

This pull request fixes an issue where multiple mentions were being created for one statuses, when delivered to multiple inboxes on an instance, but only the final mention ended up actually being used. Also adds tests to make sure that a notification is received when a status is edited to include a mention.

## Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: `[ ]` -> `[x]`

If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

- [x] I/we have read the [GoToSocial contribution guidelines](https://codeberg.org/superseriousbusiness/gotosocial/src/branch/main/CONTRIBUTING.md).
- [x] I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
- [x] I/we have not leveraged AI to create the proposed changes.
- [x] I/we have performed a self-review of added code.
- [x] I/we have written code that is legible and maintainable by others.
- [x] I/we have commented the added code, particularly in hard-to-understand areas.
- [ ] I/we have made any necessary changes to documentation.
- [x] I/we have added tests that cover new code.
- [x] I/we have run tests and they pass locally with the changes.
- [x] I/we have run `go fmt ./...` and `golangci-lint run`.

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4127
Co-authored-by: tobi <tobi.smethurst@protonmail.com>
Co-committed-by: tobi <tobi.smethurst@protonmail.com>

internal/processing/workers/fromfediapi_test.go

@@ -735,6 +735,52 @@ func (suite *FromFediAPITestSuite) TestUndoAnnounce() {
 	}
 }
 
+func (suite *FromFediAPITestSuite) TestUpdateNote() {
+	var (
+		ctx            = context.Background()
+		testStructs    = testrig.SetupTestStructs(rMediaPath, rTemplatePath)
+		requestingAcct = suite.testAccounts["remote_account_2"]
+		receivingAcct  = suite.testAccounts["local_account_1"]
+	)
+	defer testrig.TearDownTestStructs(testStructs)
+
+	update := testrig.NewTestActivities(suite.testAccounts)["remote_account_2_status_1_update"]
+	statusable := update.Activity.GetActivityStreamsObject().At(0).GetActivityStreamsNote()
+	noteURI := ap.GetJSONLDId(statusable)
+
+	// Get the OG status.
+	status, err := testStructs.State.DB.GetStatusByURI(ctx, noteURI.String())
+	if err != nil {
+		suite.FailNow(err.Error())
+	}
+
+	// Process the Update.
+	err = testStructs.Processor.Workers().ProcessFromFediAPI(ctx, &messages.FromFediAPI{
+		APObjectType:   ap.ObjectNote,
+		APActivityType: ap.ActivityUpdate,
+		GTSModel:       status, // original status
+		APObject:       (ap.Statusable)(statusable),
+		Receiving:      receivingAcct,
+		Requesting:     requestingAcct,
+	})
+	suite.NoError(err)
+
+	// Wait for side effects to trigger:
+	// zork should have a mention notif.
+	if !testrig.WaitFor(func() bool {
+		_, err := testStructs.State.DB.GetNotification(
+			gtscontext.SetBarebones(ctx),
+			gtsmodel.NotificationMention,
+			receivingAcct.ID,
+			requestingAcct.ID,
+			status.ID,
+		)
+		return err == nil
+	}) {
+		suite.FailNow("timed out waiting for mention notif")
+	}
+}
+
 func TestFromFederatorTestSuite(t *testing.T) {
 	suite.Run(t, &FromFediAPITestSuite{})
 }