fix(completion): completions.external.enable config option not respected (#15443)

Fixes #15441 

# Description

Actually I made a small change to the original behavior:

```
^foo<tab>
```
will still show external commands, regardless of whether it's enabled or
not. I think that's the only thing people want to see when they press
tab with a `^` prefix.

# User-Facing Changes

# Tests + Formatting

+1

# After Submitting

Should I document that minor behavior change somewhere in GitHub.io?

---------

Co-authored-by: Yash Thakur <45539777+ysthakur@users.noreply.github.com>

crates/nu-cli/src/completions/completer.rs

@@ -486,9 +486,10 @@ impl NuCompleter {
         externals: bool,
         strip: bool,
     ) -> Vec<SemanticSuggestion> {
+        let config = self.engine_state.get_config();
         let mut command_completions = CommandCompletion {
             internals,
-            externals,
+            externals: !internals || (externals && config.completions.external.enable),
         };
         let (new_span, prefix) = strip_placeholder_if_any(working_set, &span, strip);
         let ctx = Context::new(working_set, new_span, prefix, offset);

crates/nu-cli/tests/completions/mod.rs

@@ -10,7 +10,7 @@ use nu_cli::NuCompleter;
 use nu_engine::eval_block;
 use nu_parser::parse;
 use nu_path::expand_tilde;
-use nu_protocol::{debugger::WithoutDebug, engine::StateWorkingSet, PipelineData};
+use nu_protocol::{debugger::WithoutDebug, engine::StateWorkingSet, Config, PipelineData};
 use reedline::{Completer, Suggestion};
 use rstest::{fixture, rstest};
 use support::{
@@ -367,7 +367,7 @@ export def say [
 
 /// External command only if starts with `^`
 #[test]
-fn external_commands_only() {
+fn external_commands() {
     let engine = new_external_engine();
     let mut completer = NuCompleter::new(
         Arc::new(engine),
@@ -390,6 +390,31 @@ fn external_commands_only() {
     match_suggestions(&expected, &suggestions);
 }
 
+/// Disable external commands except for those start with `^`
+#[test]
+fn external_commands_disabled() {
+    let mut engine = new_external_engine();
+
+    let mut config = Config::default();
+    config.completions.external.enable = false;
+    engine.set_config(config);
+
+    let stack = nu_protocol::engine::Stack::new();
+    let mut completer = NuCompleter::new(Arc::new(engine), Arc::new(stack));
+    let completion_str = "ls; ^sleep";
+    let suggestions = completer.complete(completion_str, completion_str.len());
+    #[cfg(windows)]
+    let expected: Vec<_> = vec!["sleep.exe"];
+    #[cfg(not(windows))]
+    let expected: Vec<_> = vec!["sleep"];
+    match_suggestions(&expected, &suggestions);
+
+    let completion_str = "sleep";
+    let suggestions = completer.complete(completion_str, completion_str.len());
+    let expected: Vec<_> = vec!["sleep"];
+    match_suggestions(&expected, &suggestions);
+}
+
 /// Which completes both internals and externals
 #[test]
 fn which_command_completions() {