More `MessagePart` types

[alexmojaki]

Area(s)

area:gen-ai

What's missing?

Assuming that #2179 gets merged, we will need many more MessagePart types to cover the different things that can be sent to and received from a model in a message, e.g. reasoning, images, and builtin tool calls. Some of these problems may also be covered by extending existing parts or just agreeing to use them.

Describe the solution you'd like

Separate issues should be created for each use case and linked to from here to make the conversation cohesive.

[alexmojaki]

Multimedia: images, inputs, documents, videos: #1556

[alexmojaki]

Citations within parts of type text: #1952 (comment)

[alexmojaki]

Builtin tools such as code interpreters: #2585