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_test.go

@@ -30,9 +30,17 @@ type mcpToolResult struct {
 	} `json:"error,omitempty"`
 }
 
+// writeMCPResponse mirrors docmark's REAL response framing (empirically
+// confirmed against the live server): Content-Type: text/event-stream,
+// body is SSE-framed ("event: message\r\ndata: {...}\r\n\r\n"), not bare
+// JSON -- inherent to MCP Streamable-HTTP, independent of stateless_http.
 func writeMCPResponse(w http.ResponseWriter, body mcpToolResult) {
-	w.Header().Set("Content-Type", "application/json")
-	_ = json.NewEncoder(w).Encode(body)
+	payload, _ := json.Marshal(body)
+	w.Header().Set("Content-Type", "text/event-stream")
+	w.WriteHeader(http.StatusOK)
+	_, _ = w.Write([]byte("event: message\r\ndata: "))
+	_, _ = w.Write(payload)
+	_, _ = w.Write([]byte("\r\n\r\n"))
 }
 
 func TestExtractViaDocmark_Success(t *testing.T) {