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/markdown_parser.go

@@ -49,10 +49,11 @@ const GoMarkdown = "go_markdown"
 var ssrfAllowLoopback bool
 
 type MarkdownParser struct {
-	libType      string
-	ParseMethod  string
-	OutputFormat string
-	VLM          map[string]any
+	libType            string
+	ParseMethod        string
+	OutputFormat       string
+	VLM                map[string]any
+	FlattenMediaToText bool
 }
 
 func NewMarkdownParser(libType string) (*MarkdownParser, error) {
@@ -79,6 +80,9 @@ func (p *MarkdownParser) ConfigureFromSetup(setup map[string]any) {
 	if v, ok := setup["vlm"].(map[string]any); ok {
 		p.VLM = v
 	}
+	if v, ok := setup["flatten_media_to_text"].(bool); ok {
+		p.FlattenMediaToText = v
+	}
 }
 
 // ParseWithResult implements ParseResultProducer (plan §6.5) and
@@ -94,7 +98,7 @@ func (p *MarkdownParser) ParseWithResult(ctx context.Context, filename string, d
 	rawText := string(data)
 
 	var items []map[string]any
-	walkMarkdownBlocksWithImages(doc, rawText, &items)
+	walkMarkdownBlocksWithImages(doc, rawText, &items, p.FlattenMediaToText)
 	if items == nil {
 		items = []map[string]any{{"text": "", "doc_type_kwd": "text"}}
 	}
@@ -123,8 +127,10 @@ func markdownNew() *parser.Parser {
 // emitted with their text. When a block contains a markdown image
 // reference (![alt](src)), the image data is resolved via
 // resolveMarkdownImage and the item carries `doc_type_kwd: "image"`
-// together with the base64-encoded image payload.
-func walkMarkdownBlocksWithImages(doc ast.Node, rawText string, out *[]map[string]any) {
+// together with the base64-encoded image payload. When flatten is
+// true, all items are forced to doc_type_kwd="text" (mirrors Python
+// parser.py:1034 flatten_media_to_text).
+func walkMarkdownBlocksWithImages(doc ast.Node, rawText string, out *[]map[string]any, flatten bool) {
 	for _, child := range doc.GetChildren() {
 		var ckType string
 		var docTypeKwd string
@@ -166,9 +172,13 @@ func walkMarkdownBlocksWithImages(doc ast.Node, rawText string, out *[]map[strin
 		// Detect markdown image references in the raw source text
 		// that corresponds to this block. When found, resolve the
 		// image data so downstream vision enhancement can describe it.
+		// When flatten is true, keep doc_type_kwd="text" (Python
+		// parser.py:1034: flatten_media_to_text overrides image).
 		if imgData, imgFound := resolveMarkdownImage(txt, rawText); imgFound && imgData != "" {
-			item["doc_type_kwd"] = "image"
 			item["image"] = imgData
+			if !flatten {
+				item["doc_type_kwd"] = "image"
+			}
 		}
 
 		*out = append(*out, item)