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/chunk_test.go

@@ -0,0 +1,89 @@
+//go:build cgo
+
+package parser
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"ragflow/internal/deepdoc/parser/pdf/tools"
+)
+
+// TestParse_ChunkEquivalence verifies that chunked processing produces
+// the same output as processing all pages at once. Uses chunkSize=1
+// (every page is its own chunk) on a multi-page fixture to maximize
+// chunk boundary stress.
+func TestParse_ChunkEquivalence(t *testing.T) {
+	data, err := readTestPDF(t, "03_multipage.pdf")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	parse := func(chunkSize int) *ParseResult {
+		eng, err := NewEngine(data)
+		if err != nil {
+			t.Fatal(err)
+		}
+		defer eng.Close()
+		cfg := DefaultParserConfig()
+		cfg.ChunkSize = chunkSize
+		p := NewParser(cfg, &MockDocAnalyzer{Healthy: true, Model: ModelSaas})
+		result, err := p.Parse(context.Background(), eng)
+		if err != nil {
+			t.Fatal(err)
+		}
+		return result
+	}
+
+	// No chunking (all pages at once).
+	full := parse(9999)
+	// Aggressive chunking (1 page per chunk).
+	chunked := parse(1)
+
+	// Compare section counts.
+	if len(full.Sections) != len(chunked.Sections) {
+		t.Logf("section count: full=%d chunked=%d (small diff acceptable at chunk boundaries)",
+			len(full.Sections), len(chunked.Sections))
+	}
+
+	// Compare text content via CharSimilarity.
+	fullText := sectionsText(full.Sections)
+	chunkedText := sectionsText(chunked.Sections)
+	charSim := tools.CharSimilarity(fullText, chunkedText)
+	t.Logf("CharSimilarity: %.1f%%", charSim)
+	if charSim < 95 {
+		t.Errorf("chunk equivalence too low: CharSim=%.1f%% (want >= 95%%)", charSim)
+	}
+
+	// Compare metrics (should be identical or very close).
+	t.Logf("Metrics: full=%+v chunked=%+v", full.Metrics, chunked.Metrics)
+	if full.Metrics.BoxesInitial != chunked.Metrics.BoxesInitial {
+		t.Errorf("BoxesInitial: full=%d chunked=%d",
+			full.Metrics.BoxesInitial, chunked.Metrics.BoxesInitial)
+	}
+
+	// Bug fix regression: PageImages must survive chunked merge.
+	if len(full.PageImages) == 0 {
+		t.Error("full parse: PageImages should not be empty (3-page document)")
+	}
+	if len(chunked.PageImages) == 0 {
+		t.Error("chunked parse: PageImages should be preserved across chunks")
+	}
+}
+
+func readTestPDF(t *testing.T, name string) ([]byte, error) {
+	t.Helper()
+	return os.ReadFile(filepath.Join("testdata", "pdfs", name))
+}
+
+func sectionsText(sections []Section) string {
+	var sb strings.Builder
+	for _, s := range sections {
+		sb.WriteString(s.Text)
+		sb.WriteByte('\n')
+	}
+	return sb.String()
+}