fix(marker): reject reserved global-prefix names only on create

The global-prefix reservation belongs on the create path, where it keeps
user-chosen names disjoint from inherited global-{name} markers. It was
implemented as a field_validator on MarkerCreate / MarkerDefinitionCreate,
which also back the PUT update bodies — so updating an inherited marker
while echoing its name in the body tripped the validator with a generic
422 before the controller could return the actionable 409 ("inherited,
use the marker-definitions API").

Drop the schema validators and enforce the prefix in the two create routes
(ControllerError -> 409). PUT no longer validates the body name (ignored
anyway — the target is the {name} path param), so editing an inherited
marker now reaches the controller's inheritance guard and returns the
clear 409. The name-format regex stays on the schema (still 422).

gns3server/api/routes/controller/links.py

@@ -458,6 +458,8 @@ async def create_marker(
     # uuid suffix avoids the collision that `marker-{link.id[:8]}` alone would
     # cause on the second anonymous marker on the same link (start_marker
     # rejects duplicate names).
+    if marker_data.name and marker_data.name.lower().startswith("global"):
+        raise ControllerError('Names starting with "global" are reserved for inherited markers')
     name = marker_data.name or f"marker-{link.id[:8]}-{uuid4().hex[:4]}"
     await link.start_marker(
         name=name,