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/controller/udp_link.py

@@ -17,7 +17,7 @@
 
 
 from .controller_error import ControllerError, ControllerNotFoundError
-from .link import Link
+from .link import Link, _UNSET
 from .node_types import BUILTIN_NODE_TYPES
 from gns3server.utils.packet_filter_validation import validate_bpf_syntax, FilterValidationError
 
@@ -430,7 +430,7 @@ class UDPLink(Link):
         self._project.emit_notification("link.updated", self.asdict())
         self._project.dump()
 
-    async def update_marker(self, name, bpf=None, tag=None, enabled=None, direction=None, color=None, highlight_duration=None, inherited=False):
+    async def update_marker(self, name, bpf=None, tag=None, enabled=None, direction=_UNSET, color=None, highlight_duration=None, inherited=False):
         """
         Update an existing marker's BPF/tag/enabled/color. Any change pushes via
         ``self.update()``; uBridge picks up the new params on the next NIO
@@ -470,8 +470,8 @@ class UDPLink(Link):
             marker_info["color"] = color
         if highlight_duration is not None:
             marker_info["highlight_duration"] = highlight_duration
-        if direction is not None:
-            marker_info["direction"] = direction
+        if direction is not _UNSET:
+            marker_info["direction"] = direction  # None = clear back to both directions
 
         if self._created:
             await self.update()