Make LanguageModel::use_any_tool return a stream of chunks (#16262)

This PR is a refactor to pave the way for allowing the user to view and
edit workflow step resolutions. I've made tool calls work more like
normal streaming completions for all providers. The `use_any_tool`
method returns a stream of strings (which contain chunks of JSON). I've
also done some minor cleanup of language model providers in general,
removing the duplication around handling streaming responses.

Release Notes:

- N/A

crates/language_model/src/provider/ollama.rs

@@ -6,7 +6,6 @@ use ollama::{
     get_models, preload_model, stream_chat_completion, ChatMessage, ChatOptions, ChatRequest,
     ChatResponseDelta, OllamaToolCall,
 };
-use serde_json::Value;
 use settings::{Settings, SettingsStore};
 use std::{sync::Arc, time::Duration};
 use ui::{prelude::*, ButtonLike, Indicator};
@@ -311,7 +310,7 @@ impl LanguageModel for OllamaLanguageModel {
         tool_description: String,
         schema: serde_json::Value,
         cx: &AsyncAppContext,
-    ) -> BoxFuture<'static, Result<serde_json::Value>> {
+    ) -> BoxFuture<'static, Result<BoxStream<'static, Result<String>>>> {
         use ollama::{OllamaFunctionTool, OllamaTool};
         let function = OllamaFunctionTool {
             name: tool_name.clone(),
@@ -324,23 +323,19 @@ impl LanguageModel for OllamaLanguageModel {
         self.request_limiter
             .run(async move {
                 let response = response.await?;
-                let ChatMessage::Assistant {
-                    tool_calls,
-                    content,
-                } = response.message
-                else {
+                let ChatMessage::Assistant { tool_calls, .. } = response.message else {
                     bail!("message does not have an assistant role");
                 };
                 if let Some(tool_calls) = tool_calls.filter(|calls| !calls.is_empty()) {
                     for call in tool_calls {
                         let OllamaToolCall::Function(function) = call;
                         if function.name == tool_name {
-                            return Ok(function.arguments);
+                            return Ok(futures::stream::once(async move {
+                                Ok(function.arguments.to_string())
+                            })
+                            .boxed());
                         }
                     }
-                } else if let Ok(args) = serde_json::from_str::<Value>(&content) {
-                    // Parse content as arguments.
-                    return Ok(args);
                 } else {
                     bail!("assistant message does not have any tool calls");
                 };