Fix `generate_init_message` for Multimodal Messages (#2124)

* multimodal carryover

* adds mm carryover tests

* more tests + cleanup code

* check content instead

* beibin suggestion

* cleanup

* fix async

* use deepcopy

* handle carryover method

* remove content copy

* sonichi suggestions

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Co-authored-by: Beibin Li <BeibinLi@users.noreply.github.com>
Co-authored-by: Chi Wang <wang.chi@microsoft.com>