Fix SSE connection close header for Go HTTP clients

Change SSE response header from connection: keep-alive to
connection: close. Go HTTP clients (opencode) wait for the server
to close the connection, causing hangs. Python SDK was unaffected
because it closes after seeing data: [DONE].

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: antoinezambelli

src/forge/proxy/server.py

@@ -286,7 +286,7 @@ async def _send_sse(send: Any, chunks: list[bytes]) -> None:
         "headers": [
             [b"content-type", b"text/event-stream"],
             [b"cache-control", b"no-cache"],
-            [b"connection", b"keep-alive"],
+            [b"connection", b"close"],
         ],
     })
     for chunk in chunks: