fix "keep_first_message" to make sure messages are in correct order,

[milkmeat]

Why are these changes needed?

When "keep_first_message" is false, sometimes the truncated message list is in wrong order.
This fix inserts messages are in correct order,

Related issue number

#3178 (comment)

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• [y ] I've made sure all auto checks have passed.

[ekzhu]

Looks good to me. @WaelKarkoub FYI