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/python_char_adapter.go

@@ -0,0 +1,90 @@
+package parser
+
+import (
+	"encoding/json"
+	"fmt"
+	"image"
+	"os"
+)
+
+// PythonCharEngine implements PDFEngine by loading chars from a
+// charspy/{pdf}.json file exported by dump_py_results.py.
+// It is used for pipeline parity testing — same input chars as Python,
+// so any difference in pipeline output is a Go pipeline logic bug.
+type PythonCharEngine struct {
+	chars map[int][]TextChar // pageNum → chars
+	pages int
+}
+
+// LoadPythonChars loads chars from a charspy/{name}.json file.
+func LoadPythonChars(jsonPath string) (*PythonCharEngine, error) {
+	data, err := os.ReadFile(jsonPath)
+	if err != nil {
+		return nil, fmt.Errorf("read charspy json: %w", err)
+	}
+	var wrapper struct {
+		Pages [][]struct {
+			Text     string  `json:"text"`
+			X0       float64 `json:"x0"`
+			X1       float64 `json:"x1"`
+			Top      float64 `json:"top"`
+			Bottom   float64 `json:"bottom"`
+			FontName string  `json:"fontname"`
+			Size     float64 `json:"size"`
+		} `json:"pages"`
+	}
+	if err := json.Unmarshal(data, &wrapper); err != nil {
+		return nil, fmt.Errorf("parse charspy json: %w", err)
+	}
+
+	chars := make(map[int][]TextChar, len(wrapper.Pages))
+	for pg, pageChars := range wrapper.Pages {
+		result := make([]TextChar, len(pageChars))
+		for i, c := range pageChars {
+			result[i] = TextChar{
+				Text:       c.Text,
+				X0:         c.X0,
+				X1:         c.X1,
+				Top:        c.Top,
+				Bottom:     c.Bottom,
+				FontName:   c.FontName,
+				FontSize:   c.Size,
+				PageNumber: pg,
+			}
+		}
+		chars[pg] = result
+	}
+	return &PythonCharEngine{chars: chars, pages: len(wrapper.Pages)}, nil
+}
+
+// ExtractChars returns all characters for the given page (0-indexed).
+func (e *PythonCharEngine) ExtractChars(pageNum int) ([]TextChar, error) {
+	if pageNum < 0 || pageNum >= e.pages {
+		return nil, fmt.Errorf("page %d out of range [0, %d)", pageNum, e.pages)
+	}
+	return e.chars[pageNum], nil
+}
+
+// RenderPage returns a 1x1 placeholder PNG (not used in parity tests).
+func (e *PythonCharEngine) RenderPage(pageNum int, dpi float64) ([]byte, error) {
+	return nil, fmt.Errorf("PythonCharEngine: RenderPage not supported")
+}
+
+// RenderPageImage returns a 1x1 placeholder image (not used in parity tests).
+func (e *PythonCharEngine) RenderPageImage(pageNum int, dpi float64) (image.Image, error) {
+	return nil, fmt.Errorf("PythonCharEngine: RenderPageImage not supported")
+}
+
+// PageCount returns the number of pages.
+func (e *PythonCharEngine) PageCount() (int, error) {
+	return e.pages, nil
+}
+
+// RawData returns nil — this engine only supplies pre-loaded chars
+// for pipeline parity tests and does not hold PDF bytes.
+func (e *PythonCharEngine) RawData() []byte { return nil }
+
+// Close is a no-op.
+func (e *PythonCharEngine) Close() error {
+	return nil
+}