fix: change codex/sandbox-state/update from a notification to a request (#8142)

Historically, `accept_elicitation_for_prompt_rule()` was flaky because
we were using a notification to update the sandbox followed by a `shell`
tool request that we expected to be subject to the new sandbox config,
but because [rmcp](https://crates.io/crates/rmcp) MCP servers delegate
each incoming message to a new Tokio task, messages are not guaranteed
to be processed in order, so sometimes the `shell` tool call would run
before the notification was processed.

Prior to this PR, we relied on a generous `sleep()` between the
notification and the request to reduce the change of the test flaking
out.

This PR implements a proper fix, which is to use a _request_ instead of
a notification for the sandbox update so that we can wait for the
response to the sandbox request before sending the request to the
`shell` tool call. Previously, `rmcp` did not support custom requests,
but I fixed that in
https://github.com/modelcontextprotocol/rust-sdk/pull/590, which made it
into the `0.12.0` release (see #8288).

This PR updates `shell-tool-mcp` to expect
`"codex/sandbox-state/update"` as a _request_ instead of a notification
and sends the appropriate ack. Note this behavior is tied to our custom
`codex/sandbox-state` capability, which Codex honors as an MCP client,
which is why `core/src/mcp_connection_manager.rs` had to be updated as
part of this PR, as well.

This PR also updates the docs at `shell-tool-mcp/README.md`.

codex-rs/exec-server/tests/common/lib.rs

@@ -1,4 +1,4 @@
-use codex_core::MCP_SANDBOX_STATE_NOTIFICATION;
+use codex_core::MCP_SANDBOX_STATE_METHOD;
 use codex_core::SandboxState;
 use codex_core::protocol::SandboxPolicy;
 use rmcp::ClientHandler;
@@ -7,10 +7,12 @@ use rmcp::RoleClient;
 use rmcp::Service;
 use rmcp::model::ClientCapabilities;
 use rmcp::model::ClientInfo;
+use rmcp::model::ClientRequest;
 use rmcp::model::CreateElicitationRequestParam;
 use rmcp::model::CreateElicitationResult;
-use rmcp::model::CustomNotification;
+use rmcp::model::CustomRequest;
 use rmcp::model::ElicitationAction;
+use rmcp::model::ServerResult;
 use rmcp::service::RunningService;
 use rmcp::transport::ConfigureCommandExt;
 use rmcp::transport::TokioChildProcess;
@@ -82,7 +84,7 @@ pub async fn notify_readable_sandbox<P, S>(
     sandbox_cwd: P,
     codex_linux_sandbox_exe: Option<PathBuf>,
     service: &RunningService<RoleClient, S>,
-) -> anyhow::Result<()>
+) -> anyhow::Result<ServerResult>
 where
     P: AsRef<Path>,
     S: Service<RoleClient> + ClientHandler,
@@ -92,14 +94,14 @@ where
         codex_linux_sandbox_exe,
         sandbox_cwd: sandbox_cwd.as_ref().to_path_buf(),
     };
-    send_sandbox_notification(sandbox_state, service).await
+    send_sandbox_state_update(sandbox_state, service).await
 }
 
 pub async fn notify_writable_sandbox_only_one_folder<P, S>(
     writable_folder: P,
     codex_linux_sandbox_exe: Option<PathBuf>,
     service: &RunningService<RoleClient, S>,
-) -> anyhow::Result<()>
+) -> anyhow::Result<ServerResult>
 where
     P: AsRef<Path>,
     S: Service<RoleClient> + ClientHandler,
@@ -119,24 +121,23 @@ where
         codex_linux_sandbox_exe,
         sandbox_cwd: writable_folder.as_ref().to_path_buf(),
     };
-    send_sandbox_notification(sandbox_state, service).await
+    send_sandbox_state_update(sandbox_state, service).await
 }
 
-async fn send_sandbox_notification<S>(
+async fn send_sandbox_state_update<S>(
     sandbox_state: SandboxState,
     service: &RunningService<RoleClient, S>,
-) -> anyhow::Result<()>
+) -> anyhow::Result<ServerResult>
 where
     S: Service<RoleClient> + ClientHandler,
 {
-    let sandbox_state_notification = CustomNotification::new(
-        MCP_SANDBOX_STATE_NOTIFICATION,
-        Some(serde_json::to_value(sandbox_state)?),
-    );
-    service
-        .send_notification(sandbox_state_notification.into())
+    let response = service
+        .send_request(ClientRequest::CustomRequest(CustomRequest::new(
+            MCP_SANDBOX_STATE_METHOD,
+            Some(serde_json::to_value(sandbox_state)?),
+        )))
         .await?;
-    Ok(())
+    Ok(response)
 }
 
 pub struct InteractiveClient {