GH-27059: Rewrite error messages about msg length (#29252)

Co-authored-by: Mattermost Build <build@mattermost.com>
Co-authored-by: Ben Schumacher <ben.schumacher@mattermost.com>

server/i18n/en.json

@@ -9277,8 +9277,8 @@
     "translation": "Invalid file ids."
   },
   {
-    "id": "model.draft.is_valid.msg.app_error",
-    "translation": "Invalid message."
+    "id": "model.draft.is_valid.message_length.app_error",
+    "translation": "Draft Message property is longer than the maximum permitted length."
   },
   {
     "id": "model.draft.is_valid.priority.app_error",
@@ -9729,8 +9729,8 @@
     "translation": "Invalid Id."
   },
   {
-    "id": "model.post.is_valid.msg.app_error",
-    "translation": "Invalid message."
+    "id": "model.post.is_valid.message_length.app_error",
+    "translation": "Post Message property is longer than the maximum permitted length."
   },
   {
     "id": "model.post.is_valid.original_id.app_error",

server/public/model/draft.go

@@ -28,7 +28,8 @@ type Draft struct {
 
 func (o *Draft) IsValid(maxDraftSize int) *AppError {
 	if utf8.RuneCountInString(o.Message) > maxDraftSize {
-		return NewAppError("Drafts.IsValid", "model.draft.is_valid.msg.app_error", nil, "channelid="+o.ChannelId, http.StatusBadRequest)
+		return NewAppError("Drafts.IsValid", "model.draft.is_valid.message_length.app_error",
+			map[string]any{"Length": utf8.RuneCountInString(o.Message), "MaxLength": maxDraftSize}, "channelid="+o.ChannelId, http.StatusBadRequest)
 	}
 
 	return o.BaseIsValid()

server/public/model/post.go

@@ -437,7 +437,8 @@ func (o *Post) IsValid(maxPostSize int) *AppError {
 	}
 
 	if utf8.RuneCountInString(o.Message) > maxPostSize {
-		return NewAppError("Post.IsValid", "model.post.is_valid.msg.app_error", nil, "id="+o.Id, http.StatusBadRequest)
+		return NewAppError("Post.IsValid", "model.post.is_valid.message_length.app_error",
+			map[string]any{"Length": utf8.RuneCountInString(o.Message), "MaxLength": maxPostSize}, "id="+o.Id, http.StatusBadRequest)
 	}
 
 	if utf8.RuneCountInString(o.Hashtags) > PostHashtagsMaxRunes {

server/public/model/post_test.go

@@ -59,6 +59,11 @@ func TestPostIsValid(t *testing.T) {
 	appErr = o.IsValid(maxPostSize)
 	require.NotNil(t, appErr)
 
+	// In case message property length is too long.
+	o.Message = strings.Repeat("0", maxPostSize+1)
+	appErr = o.IsValid(maxPostSize)
+	require.NotNil(t, appErr)
+
 	o.Message = strings.Repeat("0", maxPostSize)
 	appErr = o.IsValid(maxPostSize)
 	require.Nil(t, appErr)