Use filepath.Ext to detect file extension in markdown renderer

Replaces manual string splitting with filepath.Ext for determining the file extension in renderFileContentAsMarkdown. This improves accuracy, especially for files with multiple dots in their names.

pkg/cmd/agent-task/shared/log.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"path/filepath"
 	"slices"
 	"strings"
 
@@ -366,10 +367,9 @@ func stripDiffFormat(diff string) string {
 // renderFileContentAsMarkdown renders the given content as markdown
 // based on the file extension of the path.
 func renderFileContentAsMarkdown(path, content string, w io.Writer, io *iostreams.IOStreams) error {
-	parts := strings.Split(path, ".")
-	lang := parts[len(parts)-1]
+	lang := filepath.Ext(filepath.ToSlash(path))
 
-	if lang == "md" {
+	if lang == ".md" {
 		return renderRawMarkdown(content, w, io)
 	}