mise: add bash completion to bash integration

The bash integration previously only ran `mise activate bash`, so tab
completion for `mise` was never set up. Source the generated completion
script via `mise completion bash --include-bash-completion-lib`; the
flag self-bundles the bash-completion helpers so it also works on
systems whose bash-completion predates `_comp_initialize`.

Fixes #8539

modules/programs/mise.nix

@@ -111,9 +111,17 @@ in
     };
 
     programs = {
-      bash.initExtra = mkIf (cfg.enableBashIntegration && cfg.package != null) ''
-        eval "$(${getExe cfg.package} activate bash)"
-      '';
+      bash.initExtra =
+        let
+          # TODO: Upstream to nixpkgs
+          bashCompletion = pkgs.runCommand "mise-bash-completion.bash" { } ''
+            ${getExe cfg.package} completion bash --include-bash-completion-lib > $out
+          '';
+        in
+        mkIf (cfg.enableBashIntegration && cfg.package != null) ''
+          eval "$(${getExe cfg.package} activate bash)"
+          source ${bashCompletion}
+        '';
 
       zsh.initContent = mkIf (cfg.enableZshIntegration && cfg.package != null) ''
         eval "$(${getExe cfg.package} activate zsh)"