Handle SSE data fields without spaces (#60575)

Fixes #60270.

## Summary

- Accept MCP HTTP SSE `data:` fields when the optional space after the
colon is omitted.
- Add a regression test covering `data:{...}` streamable HTTP responses.

## Tests

- `cargo fmt --check`
- `cargo test -p context_server
test_sse_data_field_without_space_after_colon -- --nocapture`
- `cargo test -p context_server -- --nocapture`

Release Notes:

- Fixed MCP HTTP context servers timing out when SSE data fields omit
the optional space after the colon.

---------

Co-authored-by: dino <dinojoaocosta@gmail.com>

crates/context_server/src/transport/http.rs

@@ -300,7 +300,7 @@ impl HttpTransport {
                                     data_buffer.clear();
                                 }
                                 in_message = false;
-                            } else if let Some(data) = line.strip_prefix("data: ") {
+                            } else if let Some(data) = line.strip_prefix("data:") {
                                 // Handle data lines
                                 let data = data.trim();
                                 if !data.is_empty() {
@@ -412,9 +412,13 @@ impl Drop for HttpTransport {
 mod tests {
     use super::*;
     use async_trait::async_trait;
+    use futures::FutureExt as _;
     use gpui::TestAppContext;
     use parking_lot::Mutex as SyncMutex;
-    use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
+    use std::{
+        sync::atomic::{AtomicBool, AtomicUsize, Ordering},
+        time::Duration,
+    };
 
     /// A mock token provider that returns a configurable token and tracks
     /// refresh attempts.
@@ -492,6 +496,52 @@ mod tests {
             .unwrap())
     }
 
+    #[gpui::test]
+    async fn test_sse_data_field(cx: &mut TestAppContext) {
+        for data_prefix in ["data:", "data: "] {
+            let body = format!(
+                "id:1\nevent:message\n{data_prefix}{{\"jsonrpc\":\"2.0\",\"id\":1,\"result\":{{}}}}\n\n"
+            );
+            let client = make_fake_http_client(move |_req| {
+                let body = body.clone();
+                Box::pin(async move {
+                    Ok(Response::builder()
+                        .status(200)
+                        .header("Content-Type", "text/event-stream;charset=UTF-8")
+                        .body(AsyncBody::from(body.into_bytes()))
+                        .unwrap())
+                })
+            });
+
+            let transport = HttpTransport::new(
+                client,
+                "http://mcp.example.com/mcp".to_string(),
+                HashMap::default(),
+                cx.background_executor.clone(),
+            );
+
+            transport
+                .send(r#"{"jsonrpc":"2.0","id":1,"method":"initialize"}"#.to_string())
+                .await
+                .expect("send should succeed");
+
+            let mut responses = transport.receive();
+            let next_response = responses.next().fuse();
+            let timeout = cx.background_executor.timer(Duration::from_secs(1)).fuse();
+            futures::pin_mut!(next_response, timeout);
+
+            let response = futures::select_biased! {
+                response = next_response => response.expect("expected SSE response"),
+                _ = timeout => panic!("timed out waiting for SSE response with {data_prefix:?}"),
+            };
+
+            assert_eq!(
+                response, r#"{"jsonrpc":"2.0","id":1,"result":{}}"#,
+                "unexpected SSE response for {data_prefix:?}",
+            );
+        }
+    }
+
     #[gpui::test]
     async fn test_bearer_token_attached_to_requests(cx: &mut TestAppContext) {
         // Capture the Authorization header from the request.