Trim compaction input (#10374)

Two fixes:

1. Include trailing tool output in the total context size calculation.
Otherwise when checking whether compaction should run we ignore newly
added outputs.
2. Trim trailing tool output/tool calls until we can fit the request
into the model context size. Otherwise the compaction endpoint will fail
to compact. We only trim items that can be reproduced again by the model
(tool calls, tool call outputs).

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

@@ -2,4 +2,5 @@ mod history;
 mod normalize;
 
 pub(crate) use history::ContextManager;
+pub(crate) use history::is_codex_generated_item;
 pub(crate) use history::is_user_turn_boundary;