feat(diff): Enable commenting on expanded lines in PR diffs (#35662)

Fixes #32257 
/claim #32257

Implemented commenting on unchanged lines in Pull Request diffs, lines
are accessed by expanding the diff preview. Comments also appear in the
"Files Changed" tab on the unchanged lines where they were placed.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

routers/web/repo/compare_test.go

@@ -0,0 +1,40 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package repo
+
+import (
+	"testing"
+
+	issues_model "code.gitea.io/gitea/models/issues"
+	"code.gitea.io/gitea/services/gitdiff"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestAttachCommentsToLines(t *testing.T) {
+	section := &gitdiff.DiffSection{
+		Lines: []*gitdiff.DiffLine{
+			{LeftIdx: 5, RightIdx: 10},
+			{LeftIdx: 6, RightIdx: 11},
+		},
+	}
+
+	lineComments := map[int64][]*issues_model.Comment{
+		-5: {{ID: 100, CreatedUnix: 1000}},                               // left side comment
+		10: {{ID: 200, CreatedUnix: 2000}},                               // right side comment
+		11: {{ID: 300, CreatedUnix: 1500}, {ID: 301, CreatedUnix: 2500}}, // multiple comments
+	}
+
+	attachCommentsToLines(section, lineComments)
+
+	// First line should have left and right comments
+	assert.Len(t, section.Lines[0].Comments, 2)
+	assert.Equal(t, int64(100), section.Lines[0].Comments[0].ID)
+	assert.Equal(t, int64(200), section.Lines[0].Comments[1].ID)
+
+	// Second line should have two comments, sorted by creation time
+	assert.Len(t, section.Lines[1].Comments, 2)
+	assert.Equal(t, int64(300), section.Lines[1].Comments[0].ID)
+	assert.Equal(t, int64(301), section.Lines[1].Comments[1].ID)
+}