Allow large file writes over direct exec-server WebSockets (#54135)

## Why

Direct mode serves executor requests but applied the client's 8 KiB inbound request limit, causing larger `fs/writeFile` requests to disconnect the executor.

## What changed

Use a dedicated direct WebSocket connection constructor that omits the client inbound request limit while retaining WebSocket message and frame limits.

## Testing

Add a regression test that runs the direct environment and verifies successful file writes above the client request limit using both text and binary WebSocket messages.

GitOrigin-RevId: e8e77a5e8581ab9eb98446e23b77a6da41d64868

codex-rs/exec-server/src/connection.rs

@@ -481,6 +481,22 @@ impl JsonRpcConnection {
         )
     }
 
+    /// Direct mode dials the WebSocket but serves executor requests from the
+    /// harness. Do not apply the harness client's inbound request limit here;
+    /// the WebSocket transport still enforces its message and frame limits.
+    pub(crate) fn from_direct_websocket<T, E>(stream: T, connection_label: String) -> Self
+    where
+        T: Sink<Message, Error = E> + Stream<Item = Result<Message, E>> + Unpin + Send + 'static,
+        E: std::fmt::Display + Send + 'static,
+    {
+        Self::from_websocket_stream(
+            stream,
+            connection_label,
+            /*ping_interval*/ None,
+            /*max_inbound_request_len*/ None,
+        )
+    }
+
     pub(crate) fn from_axum_websocket(stream: AxumWebSocket, connection_label: String) -> Self {
         Self::from_websocket_stream(
             stream,