Avoid double truncation (#6631)

1. Avoid double truncation by giving 10% above the tool default constant
2. Add tests that fails when const = 1

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

@@ -3,4 +3,6 @@ mod normalize;
 mod truncate;
 
 pub(crate) use history::ContextManager;
+pub(crate) use truncate::MODEL_FORMAT_MAX_BYTES;
+pub(crate) use truncate::MODEL_FORMAT_MAX_LINES;
 pub(crate) use truncate::format_output_for_model_body;