Align Go parser backends and PDF pipeline with Python (#16676)

Ports remaining Go parser wiring and PDF backends, adds tenant-aware VLM
dispatch, aligns post-processing with Python, and adds end-to-end
pipeline coverage with a generated six-page PDF.

internal/parser/parser/pdf_parser_plaintext_cgo.go

@@ -0,0 +1,38 @@
+//go:build cgo
+
+package parser
+
+import (
+	"fmt"
+
+	"ragflow/internal/deepdoc/parser/pdf/pdfoxide"
+)
+
+func parsePDFWithPlainText(filename string, data []byte, parser *PDFParser) ParseResult {
+	if len(data) == 0 {
+		return emptyPDFResult(filename)
+	}
+	doc, err := pdfoxide.OpenBytes(data)
+	if err != nil {
+		return ParseResult{Err: fmt.Errorf("parser: plain_text open: %w", err)}
+	}
+	defer doc.Close()
+
+	pageCount, err := doc.PageCount()
+	if err != nil {
+		return ParseResult{Err: fmt.Errorf("parser: plain_text page count: %w", err)}
+	}
+	items := make([]map[string]any, 0, pageCount)
+	for page := 0; page < pageCount; page++ {
+		text, err := doc.GetPageText(page)
+		if err != nil {
+			return ParseResult{Err: fmt.Errorf("parser: plain_text page %d: %w", page+1, err)}
+		}
+		items = append(items, map[string]any{
+			"text":         text,
+			"doc_type_kwd": "text",
+			"page_number":  page + 1,
+		})
+	}
+	return pdfItemsToResult(filename, items, parser.OutputFormat, pageCount)
+}