Centralize truncation in conversation history (#5652)

move the truncation logic to conversation history to use on any tool
output. This will help us in avoiding edge cases while truncating the
tool calls and mcp calls.

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

@@ -33,6 +33,7 @@ mod stream_no_completed;
 mod tool_harness;
 mod tool_parallelism;
 mod tools;
+mod truncation;
 mod unified_exec;
 mod user_notification;
 mod view_image;