Terminate stdio MCP servers on shutdown to avoid process leaks (#19753)

## Why

Several bug reports describe thread shutdown (including subagent
threads) leaving stdio MCP server processes behind. These reports all
point at the same lifecycle gap: Codex launches stdio MCP servers, but
the session-level shutdown path does not explicitly close MCP clients or
terminate the server process tree.

Fixes #12491
Fixes #12976
Fixes #18881
Fixes #19469

## History

This is best understood as a regression/coverage gap in MCP session
lifecycle management, not as stdio MCP cleanup being absent all along.
#10710 added process-group cleanup for stdio MCP servers, but that
cleanup only runs when the `RmcpClient`/transport is dropped. The older
reports (#12491 and #12976) came after that cleanup existed, which
suggests the remaining problem was that some higher-level shutdown paths
kept the MCP manager alive or replaced it without explicitly draining
clients. The newer reports (#18881 and #19469) exposed the same family
around manager replacement and shutdown.

## What changed

- Added an explicit stdio MCP process handle in `codex-rmcp-client` so
local MCP servers terminate their process group and executor-backed MCP
servers call the executor process terminator.
- Added `RmcpClient::shutdown()` and manager-level MCP shutdown draining
so session shutdown, channel-close fallback, MCP refresh, and connector
probing stop owned MCP clients.
- Added regression coverage that starts a stdio MCP server, begins an
in-flight blocking tool call, shuts down the client, and asserts the
server process exits.

## Verification

- `cargo test -p codex-rmcp-client`
- `cargo test -p codex-mcp`
- `just fix -p codex-rmcp-client`
- `just fix -p codex-mcp`
- `just fix -p codex-core`

- Manual before/after validation with a temporary repro script:
- Pre-fix binary from `HEAD^` (`fed0a8f4fa`): reproduced the leak with
surviving MCP server and child PIDs, `survivors=[77583, 77592]`,
`leaked=true`.
- Post-fix binary from this branch (`67e318148b`): verified both MCP
processes were gone after interrupting `codex exec`, `survivors=[]`,
`leaked=false`.

codex-rs/core/src/session/handlers.rs

@@ -883,6 +883,11 @@ pub async fn shutdown(sess: &Arc<Session>, sub_id: String) -> bool {
         .unified_exec_manager
         .terminate_all_processes()
         .await;
+    let mcp_shutdown = {
+        let mut manager = sess.services.mcp_connection_manager.write().await;
+        manager.begin_shutdown()
+    };
+    mcp_shutdown.await;
     sess.guardian_review_session.shutdown().await;
     info!("Shutting down Codex instance");
     let history = sess.clone_history().await;
@@ -1166,8 +1171,19 @@ pub(super) async fn submission_loop(
             break;
         }
     }
-    // Also drain cached guardian state if the submission loop exits because
-    // the channel closed without receiving an explicit shutdown op.
+    // If the submission loop exits because the channel closed without an
+    // explicit shutdown op, still run process teardown for child processes
+    // owned by this session.
+    sess.services
+        .unified_exec_manager
+        .terminate_all_processes()
+        .await;
+    let mcp_shutdown = {
+        let mut manager = sess.services.mcp_connection_manager.write().await;
+        manager.begin_shutdown()
+    };
+    mcp_shutdown.await;
+    // Also drain cached guardian state on this implicit shutdown path.
     sess.guardian_review_session.shutdown().await;
     debug!("Agent loop exited");
 }

codex-rs/core/src/session/mcp.rs

@@ -255,8 +255,11 @@ impl Session {
             *guard = cancel_token;
         }
 
-        let mut manager = self.services.mcp_connection_manager.write().await;
-        *manager = refreshed_manager;
+        let mut old_manager = {
+            let mut manager = self.services.mcp_connection_manager.write().await;
+            std::mem::replace(&mut *manager, refreshed_manager)
+        };
+        old_manager.shutdown().await;
     }
 
     pub(crate) async fn refresh_mcp_servers_if_requested(&self, turn_context: &TurnContext) {