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/agent/mcp/projects.py

@@ -176,15 +176,6 @@ def unlock_project_handler(params: dict[str, Any], gns3_ctx: dict[str, Any]) ->
     return {"message": f"Project {project_id} unlocked", "project_id": project_id}
 
 
-def load_project_handler(params: dict[str, Any], gns3_ctx: dict[str, Any]) -> dict[str, Any]:
-    path = params.get("path")
-    if not path:
-        return {"error": "path is required"}
-    conn = _get_connector(gns3_ctx)
-    result = conn.http_call("post", f"{conn.base_url}/projects/load", json_data={"path": path}).json()
-    return {"message": f"Project loaded from {path}", "project": result}
-
-
 def get_locked_project_handler(params: dict[str, Any], gns3_ctx: dict[str, Any]) -> dict[str, Any]:
     project_id = params.get("project_id")
     if not project_id: