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/link.py

@@ -30,6 +30,13 @@ import logging
 log = logging.getLogger(__name__)
 
 
+# Sentinel for "argument not passed". Distinct from None so marker/definition
+# updaters can tell "caller omitted direction" (keep current value) from
+# "caller passed direction=None" (clear it back to both directions). See
+# UDPLink.update_marker and Project.update_marker_definition.
+_UNSET = object()
+
+
 FILTERS = [
     {
         "type": "frequency_drop",
@@ -346,7 +353,7 @@ class Link:
         """
         raise NotImplementedError
 
-    async def update_marker(self, name, bpf=None, tag=None, enabled=None, direction=None):
+    async def update_marker(self, name, bpf=None, tag=None, enabled=None, direction=_UNSET):
         """
         Update an existing marker's BPF, tag, or enabled flag.