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

tests/modules/programs/mise/bash-integration.nix

@@ -1,8 +1,12 @@
-{ config, ... }:
+{ pkgs, ... }:
 {
   programs = {
     mise = {
-      package = config.lib.test.mkStubPackage { name = "mise"; };
+      package = pkgs.writeShellScriptBin "mise" ''
+        if [ "$1" = "completion" ]; then
+          echo "complete -F _mise mise"
+        fi
+      '';
       enable = true;
       enableBashIntegration = true;
     };
@@ -12,5 +16,7 @@
 
   nmt.script = ''
     assertFileRegex home-files/.bashrc 'eval "$(/nix/store/.*mise.*/bin/mise activate bash)"'
+    assertFileRegex home-files/.bashrc \
+      '^source /nix/store/[0-9a-z]*-mise-bash-completion.bash$'
   '';
 }