feat: expose data_link_type on the link_marker MCP tool (create-only)

Per-link markers on serial links need the WAN encapsulation (e.g.
DLT_C_HDLC) so the BPF compiles against the right link layer — the
REST API already accepts it, but the MCP tool never forwarded it.
Create passes it through; update ignores it (changing it would
invalidate the capture file), matching the REST schema semantics.

gns3server/agent/gns3_copilot/gns3_client/api_handlers.py

@@ -900,7 +900,7 @@ def link_marker_handler(params: dict[str, Any], gns3_ctx: dict[str, Any]) -> dic
         if not bpf:
             return {"error": "bpf is required for create action"}
         body: dict[str, Any] = {"bpf": bpf}
-        for opt in ("name", "tag", "capture_node_id", "color", "highlight_duration"):
+        for opt in ("name", "tag", "capture_node_id", "color", "highlight_duration", "data_link_type"):
             if params.get(opt) is not None:
                 body[opt] = params[opt]
         # direction: "tx"/"rx" set a one-way filter; "both"/omitted = no filter.