fix: honor parser params and image VLM system_prompt in Go ingestion (#17334)

## Summary
Fix the Go ingestion pipeline so that several parser setup switches and
the
image VLM prompt are actually honored end-to-end (previously the DSL
fields
existed but the Go code never read them).

- **DOCX** (`docx_parser.go`, `docx_postprocess.go`): read `remove_toc`
and
  `remove_header_footer`; apply to both JSON and markdown output paths
  (outline-based TOC removal with a text-heuristic fallback, plus
  header/footer section filtering).
- **HTML** (`html_parser.go`, `html_postprocess.go`, `text_toc.go`):
read
`remove_header_footer` (pre-parse strip of `<header>`/`<footer>` and
ARIA
`banner`/`contentinfo`) and `remove_toc` (post-parse
`remove_contents_table`
  heuristic).
- **Markdown** (`markdown_parser.go`): read `flatten_media_to_text` and
force
  media blocks to text when enabled.
- **Image VLM** (`media_dispatch.go`): read `system_prompt` instead of
`prompt`
  so the user-configured image VLM prompt is no longer silently dropped
  (`prompt` remains the video family key).

All flags are wired through `ConfigureFromSetup`, which the dispatch
layer
already invokes for every family, so the behavior is live rather than
dead code.

## Test plan
- New unit tests: `docx_postprocess_test.go`, `html_parser_test.go`,
`text_toc_test.go`, `markdown_parser_test.go`, `media_dispatch_test.go`.
- `bash build.sh --test ./internal/parser/parser/...
./internal/ingestion/component/...`

## Notes
- The `File` component is excluded from this migration scope.
- Relates to the Python→Go parity diff (Parser 1.8–1.11, 1.15).

internal/parser/parser/html_postprocess.go

@@ -0,0 +1,51 @@
+package parser
+
+import (
+	"bytes"
+
+	"golang.org/x/net/html"
+)
+
+// stripHTMLHeaderFooter removes <header>/<footer> elements and nodes
+// with role="banner"/role="contentinfo" from the HTML byte stream,
+// then re-serializes. Mirrors Python rag/flow/parser/utils.py:70-74
+// remove_header_footer_html_blob (BeautifulSoup decompose).
+func stripHTMLHeaderFooter(data []byte) ([]byte, error) {
+	doc, err := html.Parse(bytes.NewReader(data))
+	if err != nil {
+		return nil, err
+	}
+	removeHeaderFooterNodes(doc)
+	var buf bytes.Buffer
+	if err := html.Render(&buf, doc); err != nil {
+		return nil, err
+	}
+	return buf.Bytes(), nil
+}
+
+// removeHeaderFooterNodes walks the parsed tree and detaches every
+// <header>/<footer> element and every element whose role attribute
+// is "banner" or "contentinfo".
+func removeHeaderFooterNodes(n *html.Node) {
+	var next *html.Node
+	for child := n.FirstChild; child != nil; child = next {
+		next = child.NextSibling
+		if child.Type == html.ElementNode && isHeaderFooterNode(child) {
+			n.RemoveChild(child)
+			continue
+		}
+		removeHeaderFooterNodes(child)
+	}
+}
+
+func isHeaderFooterNode(n *html.Node) bool {
+	if n.Data == "header" || n.Data == "footer" {
+		return true
+	}
+	for _, attr := range n.Attr {
+		if attr.Key == "role" && (attr.Val == "banner" || attr.Val == "contentinfo") {
+			return true
+		}
+	}
+	return false
+}