anthropic: improve handling of multipart tool responses

[lonelycode]

fixes #1010

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[bloeys]

Just wanted to note that while this (and related commits by @lonelycode) do solve the problem and give a proper tool response, I notice tool calls are not streamed. This can be extremely helpful when tool arguments are very long.

We can either include a new 'WithToolStreamingFunc' or send tool chunks into the existing streaming function with perhaps some prefix (risk of false-positive), or just update the existing function signature (breaking change, but should be fine as we are still v0.x)