fix: Scope ExecSessionManager to Session instead of using global singleton (#2664)

The `SessionManager` in `exec_command` owns a number of
`ExecCommandSession` objects where `ExecCommandSession` has a
non-trivial implementation of `Drop`, so we want to be able to drop an
individual `SessionManager` to help ensure things get cleaned up in a
timely fashion. To that end, we should have one `SessionManager` per
session rather than one global one for the lifetime of the CLI process.

codex-rs/core/src/exec_command/mod.rs

@@ -10,5 +10,5 @@ pub use responses_api::EXEC_COMMAND_TOOL_NAME;
 pub use responses_api::WRITE_STDIN_TOOL_NAME;
 pub use responses_api::create_exec_command_tool_for_responses_api;
 pub use responses_api::create_write_stdin_tool_for_responses_api;
-pub use session_manager::SESSION_MANAGER;
+pub use session_manager::SessionManager as ExecSessionManager;
 pub use session_manager::result_into_payload;

codex-rs/core/src/exec_command/session_manager.rs

@@ -2,7 +2,6 @@ use std::collections::HashMap;
 use std::io::ErrorKind;
 use std::io::Read;
 use std::sync::Arc;
-use std::sync::LazyLock;
 use std::sync::Mutex as StdMutex;
 use std::sync::atomic::AtomicU32;
 
@@ -22,8 +21,6 @@ use crate::exec_command::exec_command_session::ExecCommandSession;
 use crate::exec_command::session_id::SessionId;
 use codex_protocol::models::FunctionCallOutputPayload;
 
-pub static SESSION_MANAGER: LazyLock<SessionManager> = LazyLock::new(SessionManager::default);
-
 #[derive(Debug, Default)]
 pub struct SessionManager {
     next_session_id: AtomicU32,