feat: parallel tool calls (#4663)

Add parallel tool calls. This is configurable at model level and tool
level

codex-rs/core/tests/suite/mod.rs

@@ -24,6 +24,7 @@ mod shell_serialization;
 mod stream_error_allows_next_turn;
 mod stream_no_completed;
 mod tool_harness;
+mod tool_parallelism;
 mod tools;
 mod unified_exec;
 mod user_notification;