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/projects.py

@@ -40,6 +40,7 @@ from uuid import UUID
 from gns3server import schemas
 from gns3server.controller import Controller
 from gns3server.controller.project import Project
+from gns3server.controller.link import _UNSET
 from gns3server.controller.controller_error import ControllerError, ControllerBadRequestError
 from gns3server.controller.import_project import import_project as import_controller_project
 from gns3server.controller.export_project import export_project as export_controller_project
@@ -293,7 +294,7 @@ async def update_marker_definition(
         name=def_name,
         bpf=def_data.bpf if def_data.bpf else None,
         tag=def_data.tag,
-        direction=def_data.direction,
+        direction=def_data.direction if "direction" in def_data.model_fields_set else _UNSET,
         color=def_data.color,
         highlight_duration=def_data.highlight_duration,
     )