assistant: Stream tool uses as structured data (#17322)

This PR adjusts the approach we use to encoding tool uses in the
completion response to use a structured format rather than simply
injecting it into the response stream as text.

In #17170 we would encode the tool uses as XML and insert them as text.
This would require then re-parsing the tool uses out of the buffer in
order to use them.

The approach taken in this PR is to make `stream_completion` return a
stream of `LanguageModelCompletionEvent`s. Each of these events can be
either text, or a tool use.

A new `stream_completion_text` method has been added to `LanguageModel`
for scenarios where we only care about textual content (currently,
everywhere that isn't the Assistant context editor).

Release Notes:

- N/A

crates/language_model/src/provider/copilot_chat.rs

@@ -24,11 +24,11 @@ use ui::{
 };
 
 use crate::settings::AllLanguageModelSettings;
-use crate::LanguageModelProviderState;
 use crate::{
     LanguageModel, LanguageModelId, LanguageModelName, LanguageModelProvider,
     LanguageModelProviderId, LanguageModelProviderName, LanguageModelRequest, RateLimiter, Role,
 };
+use crate::{LanguageModelCompletionEvent, LanguageModelProviderState};
 
 use super::open_ai::count_open_ai_tokens;
 
@@ -192,7 +192,7 @@ impl LanguageModel for CopilotChatLanguageModel {
         &self,
         request: LanguageModelRequest,
         cx: &AsyncAppContext,
-    ) -> BoxFuture<'static, Result<BoxStream<'static, Result<String>>>> {
+    ) -> BoxFuture<'static, Result<BoxStream<'static, Result<LanguageModelCompletionEvent>>>> {
         if let Some(message) = request.messages.last() {
             if message.contents_empty() {
                 const EMPTY_PROMPT_MSG: &str =
@@ -243,7 +243,13 @@ impl LanguageModel for CopilotChatLanguageModel {
             }).await
         });
 
-        async move { Ok(future.await?.boxed()) }.boxed()
+        async move {
+            Ok(future
+                .await?
+                .map(|result| result.map(LanguageModelCompletionEvent::Text))
+                .boxed())
+        }
+        .boxed()
     }
 
     fn use_any_tool(