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/text_dump_test.go

@@ -0,0 +1,89 @@
+//go:build cgo && manual
+
+package parser
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+// TestDumpTextOutput runs Parse on real PDFs and saves per-PDF text
+// to testdata/output/go/noocr/text/{pdf}.txt. Set DUMP_COUNT env to limit first N PDFs.
+func TestDumpTextOutput(t *testing.T) {
+	pdfDir := filepath.Join("testdata", "real_pdfs")
+	outDir := filepath.Join("testdata", "output", "go", "noocr", "text")
+	os.MkdirAll(outDir, 0755)
+
+	entries, err := os.ReadDir(pdfDir)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	count := len(entries)
+	if n := os.Getenv("DUMP_COUNT"); n != "" {
+		c := 0
+		for _, ch := range n {
+			c = c*10 + int(ch-'0')
+		}
+		if c > 0 && c < count {
+			count = c
+		}
+	}
+
+	totalChars := 0
+	for i, e := range entries {
+		if i >= count {
+			break
+		}
+		if e.IsDir() || !strings.HasSuffix(strings.ToLower(e.Name()), ".pdf") {
+			continue
+		}
+		name := e.Name()
+		outPath := filepath.Join(outDir, name+".txt")
+		if _, err := os.Stat(outPath); err == nil {
+			data, _ := os.ReadFile(outPath)
+			n := len(data)
+			totalChars += n
+			t.Logf("[%d/%d] %s — SKIP (%d chars)", i+1, count, name, n)
+			continue
+		}
+
+		pdfPath := filepath.Join(pdfDir, name)
+		data, err := os.ReadFile(pdfPath)
+		if err != nil {
+			t.Logf("[%d/%d] %s — read error: %v", i+1, count, name, err)
+			continue
+		}
+
+		eng, err := NewEngine(data)
+		if err != nil {
+			t.Logf("[%d/%d] %s — engine error: %v", i+1, count, name, err)
+			continue
+		}
+
+		cfg := DefaultParserConfig()
+		p := NewParser(cfg, &MockDocAnalyzer{Healthy: true, Model: ModelSaas})
+		result, err := p.Parse(context.Background(), eng)
+		eng.Close()
+		if err != nil {
+			t.Logf("[%d/%d] %s — parse error: %v", i+1, count, name, err)
+			continue
+		}
+
+		var sb strings.Builder
+		for _, s := range result.Sections {
+			sb.WriteString(s.Text)
+			sb.WriteByte('\n')
+		}
+		text := sb.String()
+		os.WriteFile(outPath, []byte(text), 0644)
+
+		totalChars += len(text)
+		t.Logf("[%d/%d] %s — %d chars", i+1, count, name, len(text))
+	}
+
+	t.Logf("Done. %d chars total. Output: %s/", totalChars, outDir)
+}