fix: Handle consecutive user messages in BedrockModel

[azaylamba]

Description

Updated BedrockModel to insert a dummy assistant message when consecutive user messages are detected, ensuring proper user/assistant role alternation required by Bedrock. Added integration tests for structured output with tool usage in Bedrock Llama models.

Related Issues

#1223

Type of Change

Bug fix

Testing

I have ran all the unit tests but I have not ran all the integration tests.

• I ran hatch run prepare

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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