fix: multi-round conversion with Bedrock Claude

[goriri]

• Tests added and passed if fixing a bug or adding a new feature.
• All code checks passed.

[gventuri]

@goriri can you provide slightly more context about this fix? I don't understand exactly why we are overriding the previous message in the conversation. Does bedrock raise an error if there are 2 user messages in a row?

[gventuri]

@goriri let me know, so we can merge :)

[goriri]

@gventuri
yes. it will complain when two user messages in a row (requiring alternating user/assistant messages). in a single message, the content can contain multiple text sections.

[gventuri]

Thanks a lot for letting me know, merging! @goriri