fix(markup): resolve and proxy srcset links of <source> elements (#39708) (#39756)

Backport #39708

Co-authored-by: Basavaraj S m <basavaraj@digiscrypt.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

models/renderhelper/repo_file_test.go

@@ -9,6 +9,7 @@ import (
 	repo_model "gitea.dev/models/repo"
 	"gitea.dev/models/unittest"
 	"gitea.dev/modules/markup/markdown"
+	"gitea.dev/modules/test"
 
 	_ "gitea.dev/modules/markup/orgmode"
 
@@ -80,6 +81,23 @@ func TestRepoFile(t *testing.T) {
 <video src="/user2/repo1/media/commit/1234/my%20dir/LINK">
 </video>`, rendered)
 	})
+
+	t.Run("PictureSource", func(t *testing.T) {
+		rctx := NewRenderContextRepoFile(t.Context(), repo1, RepoFileOptions{CurrentRefSubURL: "branch/main"}).
+			WithMarkupType(markdown.MarkupName)
+		rendered, err := testRenderString(rctx, `<picture><source media="(prefers-color-scheme: dark)" srcset="dark.svg"><img src="light.svg"></picture>`)
+		assert.NoError(t, err)
+		assert.Equal(t, test.NormalizeHTMLSpaces(`
+<p>
+	<a href="/user2/repo1/src/branch/main/light.svg" target="_blank" rel="nofollow noopener">
+		<picture>
+			<source media="(prefers-color-scheme: dark)" srcset="/user2/repo1/media/branch/main/dark.svg"/>
+			<img src="/user2/repo1/media/branch/main/light.svg"/>
+		</picture>
+	</a>
+</p>
+`), test.NormalizeHTMLSpaces(rendered))
+	})
 }
 
 func TestRepoFileOrgMode(t *testing.T) {