Improve packet filter validation: use tcpdump, handle multi-line BPF, safe project load

Changes:
- Replace tshark BPF validation with tcpdump -d (calls pcap_compile
  internally like ubridge, returns instantly without waiting for traffic)
- Support multi-line BPF expressions: split on newlines and validate
  each line individually
- Always validate, never save invalid filters on error
- Drop invalid filters during project load with warning (prevents
  old topologies with bad filters from failing to open)
- Simplify test cases (no longer depend on tshark availability)

gns3server/controller/link.py

@@ -583,7 +583,7 @@ class Link:
                 "suspend": self._suspended,
                 "show_filters_icon": getattr(self, '_show_filters_icon', True),
             }
-        return {
+        result = {
             "nodes": res,
             "link_id": self._id,
             "project_id": self._project.id,
@@ -598,3 +598,4 @@ class Link:
             "wireshark": self._wireshark,
             "show_filters_icon": getattr(self, '_show_filters_icon', True),
         }
+        return result