marker: support clearing direction via explicit null / "both"

direction was settable but not clearable: once a marker or project-level
definition had direction=tx/rx, no update path could return it to "both
directions", and the definition-sync fan-out silently kept stale values on
every inherited copy.

Introduce a _UNSET sentinel (link.py) distinct from None so updaters can
tell "caller omitted direction" (preserve) from "caller passed None"
(clear). Thread it through UDPLink.update_marker and
Project.update_marker_definition; the two REST routes use Pydantic v2
model_fields_set to translate an explicit JSON null into the sentinel.

MCP follows with a "both" token: link_marker / marker_definition handlers
map direction="both" to a null in the REST body (tri-state: omit=preserve,
tx/rx=set, both=clear), and the tool descriptions/docstrings document it.

Backward compatible: omitting direction or passing tx/rx behaves exactly
as before; only an explicit null / "both" clears.

gns3server/api/routes/controller/links.py

@@ -32,7 +32,7 @@ from uuid import UUID, uuid4
 from gns3server.controller import Controller
 from gns3server.controller.controller_error import ControllerError
 from gns3server.db.repositories.rbac import RbacRepository
-from gns3server.controller.link import Link
+from gns3server.controller.link import Link, _UNSET
 from gns3server.utils.http_client import HTTPClient
 from gns3server.utils.port_allocator import link_id_to_port
 from gns3server.utils.websocket_to_websocket import websocket_proxy
@@ -510,7 +510,7 @@ async def update_marker(
         name=marker_name,
         bpf=marker_data.bpf if marker_data.bpf else None,
         tag=marker_data.tag,
-        direction=marker_data.direction,
+        direction=marker_data.direction if "direction" in marker_data.model_fields_set else _UNSET,
         color=marker_data.color,
         enabled=marker_data.enabled,
         highlight_duration=marker_data.highlight_duration,