fix: respect UI DEFAULT_SHOW_FULL_NAME setting in email From: headers (go-gitea#9307)

Changes email's `From` field to respect the server's `DEFAULT_SHOW_FULL_NAME` setting, bringing consistency between seeing an email from a user, opening the related issue, and seeing a different name in Forgejo's web UI.

Manually tested by varying the `DEFAULT_SHOW_FULL_NAME` server property, enabling and disabling it:
![image](/attachments/234b26b5-da8a-4220-955e-f20c61e418e8)

Fixes go-gitea#9149.

## Checklist

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### Tests

- I added test coverage for Go changes...
  - [x] in their respective `*_test.go` for unit tests.
  - [ ] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- I added test coverage for JavaScript changes...
  - [ ] in `web_src/js/*.test.js` if it can be unit tested.
  - [ ] in `tests/e2e/*.test.e2e.js` if it requires interactions with a live Forgejo server (see also the [developer guide for JavaScript testing](https://codeberg.org/forgejo/forgejo/src/branch/forgejo/tests/e2e/README.md#end-to-end-tests)).
- Performed manual testing with

### Documentation

- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [ ] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [ ] I do not want this change to show in the release notes.
- [x] I want the title to show in the release notes with a link to this pull request.
- [ ] I want the content of the `release-notes/<pull request number>.md` to be be used for the release notes instead of the title.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/9307
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: Mathieu Fenniak <mathieu@fenniak.net>
Co-committed-by: Mathieu Fenniak <mathieu@fenniak.net>

Author: mfenniak

services/mailer/mail.go

@@ -575,7 +575,7 @@ func fromDisplayName(u *user_model.User) string {
 	if setting.MailService.FromDisplayNameFormatTemplate != nil {
 		var ctx bytes.Buffer
 		err := setting.MailService.FromDisplayNameFormatTemplate.Execute(&ctx, map[string]any{
-			"DisplayName": u.DisplayName(),
+			"DisplayName": u.GetDisplayName(),
 			"AppName":     setting.AppName,
 			"Domain":      setting.Domain,
 		})

services/mailer/mail_test.go

@@ -497,31 +497,44 @@ func TestFromDisplayName(t *testing.T) {
 	defer test.MockVariableValue(&setting.MailService, &setting.Mailer{FromDisplayNameFormatTemplate: template})()
 
 	tests := []struct {
-		userDisplayName string
-		fromDisplayName string
+		userDisplayName     string
+		fromDisplayName     string
+		defaultShowFullName bool
 	}{{
-		userDisplayName: "test",
-		fromDisplayName: "test",
+		userDisplayName:     "test",
+		fromDisplayName:     "test",
+		defaultShowFullName: true,
 	}, {
-		userDisplayName: "Hi Its <Mee>",
-		fromDisplayName: "Hi Its <Mee>",
+		userDisplayName:     "Hi Its <Mee>",
+		fromDisplayName:     "Hi Its <Mee>",
+		defaultShowFullName: true,
 	}, {
-		userDisplayName: "Æsir",
-		fromDisplayName: "=?utf-8?q?=C3=86sir?=",
+		userDisplayName:     "Æsir",
+		fromDisplayName:     "=?utf-8?q?=C3=86sir?=",
+		defaultShowFullName: true,
 	}, {
-		userDisplayName: "new😀user",
-		fromDisplayName: "=?utf-8?q?new=F0=9F=98=80user?=",
+		userDisplayName:     "new😀user",
+		fromDisplayName:     "=?utf-8?q?new=F0=9F=98=80user?=",
+		defaultShowFullName: true,
+	}, {
+		userDisplayName:     "new😀user",
+		fromDisplayName:     "tmp",
+		defaultShowFullName: false,
 	}}
 
 	for _, tc := range tests {
 		t.Run(tc.userDisplayName, func(t *testing.T) {
+			defer test.MockVariableValue(&setting.UI.DefaultShowFullName, tc.defaultShowFullName)()
+
 			user := &user_model.User{FullName: tc.userDisplayName, Name: "tmp"}
 			got := fromDisplayName(user)
 			assert.Equal(t, tc.fromDisplayName, got)
 		})
 	}
 
 	t.Run("template with all available vars", func(t *testing.T) {
+		defer test.MockVariableValue(&setting.UI.DefaultShowFullName, true)()
+
 		template, err = texttmpl.New("mailFrom").Parse("{{ .DisplayName }} (by {{ .AppName }} on [{{ .Domain }}])")
 		require.NoError(t, err)
 		defer test.MockVariableValue(&setting.MailService, &setting.Mailer{FromDisplayNameFormatTemplate: template})()