Add configurable low-speed timeout for OpenAI provider (#11668)

This PR adds a setting to allow configuring the low-speed timeout for
the Assistant when using the OpenAI provider.

The `low_speed_timeout_in_seconds` accepts a number of seconds that the
HTTP client can go below a minimum speed limit (currently set to 100
bytes/second) before it times out.

```json
{
  "assistant": {
    "version": "1",
    "provider": { "name": "openai", "low_speed_timeout_in_seconds": 60 }
  },
}
```

This should help the case where the `openai` provider is being used with
a local model that requires higher timeouts.

Issue: https://github.com/zed-industries/zed/issues/9913

Release Notes:

- Added a `low_speed_timeout_in_seconds` setting to the Assistant's
OpenAI provider
([#9913](https://github.com/zed-industries/zed/issues/9913)).

crates/open_ai/src/open_ai.rs

@@ -1,7 +1,9 @@
 use anyhow::{anyhow, Context, Result};
 use futures::{io::BufReader, stream::BoxStream, AsyncBufReadExt, AsyncReadExt, StreamExt};
+use isahc::config::Configurable;
 use serde::{Deserialize, Serialize};
 use serde_json::{Map, Value};
+use std::time::Duration;
 use std::{convert::TryFrom, future::Future};
 use util::http::{AsyncBody, HttpClient, Method, Request as HttpRequest};
 
@@ -206,14 +208,20 @@ pub async fn stream_completion(
     api_url: &str,
     api_key: &str,
     request: Request,
+    low_speed_timeout: Option<Duration>,
 ) -> Result<BoxStream<'static, Result<ResponseStreamEvent>>> {
     let uri = format!("{api_url}/chat/completions");
-    let request = HttpRequest::builder()
+    let mut request_builder = HttpRequest::builder()
         .method(Method::POST)
         .uri(uri)
         .header("Content-Type", "application/json")
-        .header("Authorization", format!("Bearer {}", api_key))
-        .body(AsyncBody::from(serde_json::to_string(&request)?))?;
+        .header("Authorization", format!("Bearer {}", api_key));
+
+    if let Some(low_speed_timeout) = low_speed_timeout {
+        request_builder = request_builder.low_speed_timeout(100, low_speed_timeout);
+    };
+
+    let request = request_builder.body(AsyncBody::from(serde_json::to_string(&request)?))?;
     let mut response = client.send(request).await?;
     if response.status().is_success() {
         let reader = BufReader::new(response.into_body());