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/renderer.go

@@ -0,0 +1,38 @@
+package parser
+
+import (
+	"image"
+	"reflect"
+)
+
+// renderFn is the active page-rendering function.  It defaults to
+// fallbackRender (pure Go, engine-provided RenderPageImage).  When
+// pdfium is available (*_cgo build), renderer_pdfium.go replaces it
+// with pdfiumRender via its init().
+var renderFn = fallbackRender
+
+// renderPageToImage renders a page at 216 DPI for downstream DLA/TSR/OCR.
+func renderPageToImage(engine PDFEngine, pageNum int) (image.Image, error) {
+	return renderFn(engine, pageNum)
+}
+
+// fallbackRender uses the engine's own RenderPageImage (no C dependency).
+func fallbackRender(engine PDFEngine, pageNum int) (image.Image, error) {
+	img, err := engine.RenderPageImage(pageNum, dlaDPI)
+	if err != nil {
+		return nil, err
+	}
+	// Guard against typed-nil (e.g. (*image.RGBA)(nil) returned as non-nil
+	// interface).  The plain img==nil check misses that case.
+	if img == nil || reflect.ValueOf(img).IsNil() {
+		return nil, ErrNoPDFData
+	}
+	return img, nil
+}
+
+// ErrNoPDFData is returned when the engine has no raw PDF bytes to render.
+var ErrNoPDFData = &pdfError{"engine has no raw PDF data"}
+
+type pdfError struct{ msg string }
+
+func (e *pdfError) Error() string { return e.msg }