fix: resolve import paths, remove async context managers, clean up CLI

- Change absolute imports to relative imports in gnmi module
- Remove async context manager methods (pygnmi is synchronous)
- Remove unimplemented --depth option from CLI
- Update documentation to reflect sync-only usage

src/gnmi/client.py

@@ -159,14 +159,6 @@ class GNMIClient:
                 pass  # Ignore cleanup errors
             self._client = None
 
-    async def __aenter__(self) -> GNMIClient:
-        """Enter async context manager."""
-        return self.__enter__()
-
-    async def __aexit__(self, exc_type, exc_val, exc_tb) -> None:
-        """Exit async context manager."""
-        self.__exit__(exc_type, exc_val, exc_tb)
-
     # =========================================================================
     # gNMI Operations
     # =========================================================================
@@ -288,7 +280,7 @@ class GNMIClient:
             try:
                 value = json.loads(value)
             except json.JSONDecodeError:
-                pass  # Keep as string if not valid JSON
+                pass  # Keep as string - not valid JSON, will be set as string value
 
         try:
             if operation == "delete":