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_nocgo.go

@@ -0,0 +1,12 @@
+//go:build !cgo
+
+package parser
+
+import "fmt"
+
+func parsePDFWithPlainText(filename string, data []byte, parser *PDFParser) ParseResult {
+	if len(data) == 0 {
+		return emptyPDFResult(filename)
+	}
+	return ParseResult{Err: fmt.Errorf("%w: %s", ErrPDFEngineUnavailable, filename)}
+}