fix: anthropic parallel tool call results.

[ioga]

I've found that when receiving parallel tool results, Anthropic API requires the results to be grouped within the content field of a single message. If the client returns them as several role='user', content=[<TOOL_RESULT>] messages, Anthropic API will end up return an error like this:

anthropic.BadRequestError: Error code: 400 - {'type': 'error', 'error': {'type': 'invalid_request_error', 'message': "messages.2: the following `tool_use` ids were not found in `tool_result` blocks: {'toolu_01MtMyrsycEunT8wDfk2vpAi', 'toolu_01SzMGM1XsqzqYi9gNDWkV9P'}"}}

This is similar to #562 , but is specific to Anthropic as far I can tell.

This PR groups multiple tool results within a single ModelRequest into one message which addresses the issue for me.

[TeunvdWeij]

Would be great if this could be merged soon! Thanks for your efforts.