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/__init__.py

@@ -61,7 +61,7 @@ from .projects import (
     get_project_stats_handler, update_project_handler, duplicate_project_handler,
     get_project_readme_handler, update_project_readme_handler,
     lock_project_handler, unlock_project_handler,
-    load_project_handler, get_locked_project_handler,
+    get_locked_project_handler,
 )
 from .server import (
     get_version_handler, get_statistics_handler,
@@ -1264,16 +1264,6 @@ async def project_locked(
     })
 
 
-@mcp.tool()
-async def project_load(
-    path: Annotated[str, Field(description="Filesystem path to the .gns3 project file")],
-) -> list[dict[str, Any]]:
-    """Load a project from a file path on the server's filesystem."""
-    return await asyncio.to_thread(_run_handler_sync, load_project_handler, {
-        "path": path,
-    })
-
-
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