MM-54201 Refactor mention parsing in preparation for multi-word mentions (#25030)

* MM-54201 Move ExplicitMentions to its own file and rename it (#24932)

* MM-54201 Move ExplicitMentions to its own file and rename it

* Fix vet

* MM-54201 Refactor current mention parsing into MentionParserStandard (#24936)

* MM-54201 Refactor current mention parsing into MentionParserStandard

* Fix vet

* MM-54201 Unify user and group mention parsing logic (#24937)

* MM-54201 Add MentionKeywords type

* MM-54201 Move group mentions into MentionKeywords

* Fix flaky test caused by random iteration order

* Update server/channels/app/mention_results.go

Co-authored-by: Jesse Hallam <jesse.hallam@gmail.com>

* Address feedback

---------

Co-authored-by: Jesse Hallam <jesse.hallam@gmail.com>

---------

Co-authored-by: Jesse Hallam <jesse.hallam@gmail.com>

server/channels/app/mention_results_test.go

@@ -0,0 +1,64 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package app
+
+import (
+	"testing"
+
+	"github.com/mattermost/mattermost/server/public/model"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestAddMention(t *testing.T) {
+	t.Run("should initialize Mentions and store new mentions", func(t *testing.T) {
+		m := &MentionResults{}
+
+		userID1 := model.NewId()
+		userID2 := model.NewId()
+
+		m.addMention(userID1, KeywordMention)
+		m.addMention(userID2, CommentMention)
+
+		assert.Equal(t, map[string]MentionType{
+			userID1: KeywordMention,
+			userID2: CommentMention,
+		}, m.Mentions)
+	})
+
+	t.Run("should replace existing mentions with higher priority ones", func(t *testing.T) {
+		m := &MentionResults{}
+
+		userID1 := model.NewId()
+		userID2 := model.NewId()
+
+		m.addMention(userID1, ThreadMention)
+		m.addMention(userID2, DMMention)
+
+		m.addMention(userID1, ChannelMention)
+		m.addMention(userID2, KeywordMention)
+
+		assert.Equal(t, map[string]MentionType{
+			userID1: ChannelMention,
+			userID2: KeywordMention,
+		}, m.Mentions)
+	})
+
+	t.Run("should not replace high priority mentions with low priority ones", func(t *testing.T) {
+		m := &MentionResults{}
+
+		userID1 := model.NewId()
+		userID2 := model.NewId()
+
+		m.addMention(userID1, KeywordMention)
+		m.addMention(userID2, CommentMention)
+
+		m.addMention(userID1, DMMention)
+		m.addMention(userID2, ThreadMention)
+
+		assert.Equal(t, map[string]MentionType{
+			userID1: KeywordMention,
+			userID2: CommentMention,
+		}, m.Mentions)
+	})
+}