fix(terraform): mark state partial on a rejected key update

#40512 added d.Partial(true) to resourceKeyUpdate, but it landed on the
plannedKeyMetadata error path rather than the c.UpdateKey one. The test it
shipped with drives a 400 from /key/update, which returns through the
UpdateKey branch, so TestResourceKeyUpdateFailureKeepsPriorState has been
failing on litellm_internal_staging since it merged

Set the flag on the UpdateKey branch too. Without it the SDK hands back the
diff-applied state, so a rejected update persists the planned values and the
next plan reports no changes, silently dropping the update

terraform/provider/litellm/resource_key.go

@@ -327,6 +327,7 @@ func resourceKeyUpdate(ctx context.Context, d *schema.ResourceData, m interface{
 	key.Metadata = metadata
 
 	if _, err := c.UpdateKey(key); err != nil {
+		d.Partial(true)
 		return diag.FromErr(fmt.Errorf("error updating key: %s", err))
 	}