fix(zoxide): guard cd/cdi definitions with function existence checks

Prevent errors if zoxide fails to initialize by checking that
__zoxide_z and __zoxide_zi functions exist before defining wrappers.

zsh/zoxide.zsh

@@ -6,6 +6,10 @@ if command-exists zoxide; then
   cached-eval "$(command-path zoxide)" zoxide init --cmd zox zsh
 
   # Use functions to allow regular zsh completion for cd to work.
-  cd() { __zoxide_z "$@"; }
-  alias cdi='__zoxide_zi'
+  if command-exists __zoxide_z; then
+    cd() { __zoxide_z "$@"; }
+  fi
+  if command-exists __zoxide_zi; then
+    alias cdi='__zoxide_zi'
+  fi
 fi