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/src/slash_command/docs_command.rs

@@ -3,7 +3,9 @@ use std::sync::atomic::AtomicBool;
 use std::sync::Arc;
 
 use anyhow::{anyhow, bail, Result};
-use assistant_slash_command::{SlashCommand, SlashCommandOutput, SlashCommandOutputSection};
+use assistant_slash_command::{
+    ArgumentCompletion, SlashCommand, SlashCommandOutput, SlashCommandOutputSection,
+};
 use gpui::{AppContext, Model, Task, WeakView};
 use indexed_docs::{
     IndexedDocsRegistry, IndexedDocsStore, LocalProvider, PackageName, ProviderId, RustdocIndexer,
@@ -92,7 +94,7 @@ impl SlashCommand for DocsSlashCommand {
         _cancel: Arc<AtomicBool>,
         workspace: Option<WeakView<Workspace>>,
         cx: &mut AppContext,
-    ) -> Task<Result<Vec<String>>> {
+    ) -> Task<Result<Vec<ArgumentCompletion>>> {
         self.ensure_rustdoc_provider_is_registered(workspace, cx);
 
         let indexed_docs_registry = IndexedDocsRegistry::global(cx);
@@ -107,10 +109,17 @@ impl SlashCommand for DocsSlashCommand {
             ///
             /// We will likely want to extend `complete_argument` with support for replacing just
             /// a particular range of the argument when a completion is accepted.
-            fn prefix_with_provider(provider: ProviderId, items: Vec<String>) -> Vec<String> {
+            fn prefix_with_provider(
+                provider: ProviderId,
+                items: Vec<String>,
+            ) -> Vec<ArgumentCompletion> {
                 items
                     .into_iter()
-                    .map(|item| format!("{provider} {item}"))
+                    .map(|item| ArgumentCompletion {
+                        label: item.clone(),
+                        new_text: format!("{provider} {item}"),
+                        run_command: true,
+                    })
                     .collect()
             }
 
@@ -119,7 +128,11 @@ impl SlashCommand for DocsSlashCommand {
                     let providers = indexed_docs_registry.list_providers();
                     Ok(providers
                         .into_iter()
-                        .map(|provider| provider.to_string())
+                        .map(|provider| ArgumentCompletion {
+                            label: provider.to_string(),
+                            new_text: provider.to_string(),
+                            run_command: false,
+                        })
                         .collect())
                 }
                 DocsSlashCommandArgs::SearchPackageDocs {