assistant: Improve /docs argument completions (#13876)

This PR improves the completions for arguments in the `/docs` slash
command.

We achieved this by extending the `complete_argument` method on the
`SlashCommand` trait to return a `Vec<ArgumentCompletion>` instead of a
`Vec<String>`.

In addition to the completion `label`, `ArgumentCompletion` has two new
fields that are can be used to customize the completion behavior:

- `new_text`: The actual text that will be inserted when the completion
is accepted, which may be different from what is shown by the completion
label.
- `run_command`: Whether the command is run when the completion is
accepted. This can be set to `false` to allow accepting a completion
without running the command.

Release Notes:

- N/A

---------

Co-authored-by: Antonio <antonio@zed.dev>

crates/assistant_slash_command/src/assistant_slash_command.rs

@@ -15,6 +15,16 @@ pub fn init(cx: &mut AppContext) {
     SlashCommandRegistry::default_global(cx);
 }
 
+#[derive(Debug)]
+pub struct ArgumentCompletion {
+    /// The label to display for this completion.
+    pub label: String,
+    /// The new text that should be inserted into the command when this completion is accepted.
+    pub new_text: String,
+    /// Whether the command should be run when accepting this completion.
+    pub run_command: bool,
+}
+
 pub trait SlashCommand: 'static + Send + Sync {
     fn name(&self) -> String;
     fn label(&self, _cx: &AppContext) -> CodeLabel {
@@ -28,7 +38,7 @@ pub trait SlashCommand: 'static + Send + Sync {
         cancel: Arc<AtomicBool>,
         workspace: Option<WeakView<Workspace>>,
         cx: &mut AppContext,
-    ) -> Task<Result<Vec<String>>>;
+    ) -> Task<Result<Vec<ArgumentCompletion>>>;
     fn requires_argument(&self) -> bool;
     fn run(
         self: Arc<Self>,