fix: never allow the projects directory to become a project directory

Loading a .gns3 placed directly in the projects root registered the
shared projects directory as the project path (load_project derives
the path from the file's parent directory). Deleting such an entry ran
rmtree on the projects directory itself, wiping every project until a
root-owned file stopped it, and left a zombie entry in the controller.

Three layers of protection:

- Controller.load_project() refuses a .gns3 whose parent directory is
  the projects directory; the normal subdirectory layout is unaffected
- the Project.path setter rejects the projects directory itself and
  its ancestors, closing the same hole for POST/PUT with an explicit
  path
- Project.delete() uses realpath + commonpath instead of commonprefix:
  entries whose path is the projects root are refused, and sibling
  directories sharing a string prefix (/srv/projects-evil vs
  /srv/projects) are no longer treated as inside the projects dir

Also removes the project_load MCP tool: loading by raw server
filesystem path is a footgun for automated clients; projects can still
be opened by project_id via the remaining tools.

gns3server/controller/__init__.py

@@ -745,6 +745,17 @@ class Controller:
         if not os.path.exists(path):
             raise ControllerError(f"'{path}' does not exist on the controller")
 
+        # A .gns3 file must live in its own directory: the project path is
+        # the file's parent directory. A file placed directly in the
+        # projects directory would register the shared projects root as the
+        # project directory, and deleting that project would wipe every
+        # project on the controller.
+        projects_path = os.path.realpath(self.projects_directory())
+        if os.path.realpath(os.path.dirname(path)) == projects_path:
+            raise ControllerError(
+                f"'{path}' cannot be loaded: the .gns3 file must be in its own subdirectory of '{projects_path}'"
+            )
+
         topo_data = load_topology(path)
         topo_data.pop("topology")
         topo_data.pop("version")