Refactor: migrate pdf_parser.py to golang (#16323)

### What problem does this PR solve?

Http API based on onnx model.
pdf_parser.py to golang

### Type of change

- [x] Refactoring

internal/deepdoc/parser/pdf/image_utils.go

@@ -0,0 +1,26 @@
+package parser
+
+import (
+	"bytes"
+	"image"
+	"image/jpeg"
+	"image/png"
+)
+
+// ── image encoding helpers ─────────────────────────────────────────────
+
+func encodePNG(img image.Image) ([]byte, error) {
+	var buf bytes.Buffer
+	if err := png.Encode(&buf, img); err != nil {
+		return nil, err
+	}
+	return buf.Bytes(), nil
+}
+
+func encodeJPEG(img image.Image) ([]byte, error) {
+	var buf bytes.Buffer
+	if err := jpeg.Encode(&buf, img, &jpeg.Options{Quality: 90}); err != nil {
+		return nil, err
+	}
+	return buf.Bytes(), nil
+}