fix(ingest): parse docmark's SSE-framed response, not bare JSON

Found via a real live-cluster end-to-end test (POST /ingest-path with a real
.docx against the deployed ingestion + docmark): 'decode docmark response:
invalid character e looking for beginning of value'. docmark's Streamable-HTTP
transport frames every response as SSE (event: message\r\ndata: {...}\r\n\r\n,
Content-Type: text/event-stream) -- stateless_http=True removes the need for
an initialize handshake/session ID, it does NOT change the wire framing to
bare JSON. My original client + its own test mock both assumed bare JSON,
so the unit tests passed while the real call failed.

sseDataPayload() extracts the data: line before JSON-unmarshaling; falls back
to the raw body if no SSE framing is present (forward-compatible). Test mock
(docmark_test.go) now emits the SAME framing the live server actually sends,
confirmed by directly probing docmark's live response before writing the fix.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

ingestion/internal/extract/docmark.go

@@ -97,8 +97,13 @@ func extractViaDocmark(path string) (string, error) {
 		return "", fmt.Errorf("docmark: HTTP %d: %s", resp.StatusCode, string(body))
 	}
 
+	jsonBody := body
+	if data := sseDataPayload(body); data != nil {
+		jsonBody = data
+	}
+
 	var out docmarkResponse
-	if err := json.Unmarshal(body, &out); err != nil {
+	if err := json.Unmarshal(jsonBody, &out); err != nil {
 		return "", fmt.Errorf("decode docmark response: %w", err)
 	}
 	if out.Error != nil {
@@ -114,6 +119,23 @@ func extractViaDocmark(path string) (string, error) {
 	return text, nil
 }
 
+// sseDataPayload extracts the JSON payload from an SSE-framed response body
+// ("event: message\r\ndata: {...}\r\n\r\n"). docmark's Streamable-HTTP
+// transport frames every response this way (Content-Type: text/event-stream)
+// regardless of stateless_http — that flag removes the session/initialize
+// requirement, not the SSE wire framing. Returns nil if body isn't SSE-framed
+// (e.g. a plain-JSON response, kept as a fallback for forward-compatibility).
+func sseDataPayload(body []byte) []byte {
+	const prefix = "data: "
+	for _, line := range bytes.Split(body, []byte("\n")) {
+		line = bytes.TrimRight(line, "\r")
+		if bytes.HasPrefix(line, []byte(prefix)) {
+			return bytes.TrimPrefix(line, []byte(prefix))
+		}
+	}
+	return nil
+}
+
 // fileBase returns the final path segment (like filepath.Base, kept local to
 // avoid importing path/filepath just for this one call).
 func fileBase(path string) string {