Support OpenAI and Anthropic native MCP support via MCPServerTool builtin tool

[Artui]

Closes #3100

[Artui]

@DouweM can you check it again, when you have a moment? Thanks!

[Artui]

I have also updated docs now

[DouweM]

@Artui Thanks Artur, I'll review today!

[Artui]

@DouweM i think I addressed most of the comments. I'm a bit unsure regarding passing things back to API - BuiltinToolCallPart/BuiltinToolReturnPart only has args/content passed to it, but MCP calls have additional fields on it, like server_label and such. I'm pushing an entire serialised calls there, not sure if that's correct though, but seems okay

[Artui]

@DouweM this is once again ready for review. Sorry for constant pings - I really want to start using it :D

[DouweM]

@Artui Nice work! Feel free to join our public Slack in case there's something you want to discuss more directly

[DouweM]

@Artui Check out the docs changes in https://github.com/pydantic/pydantic-ai/pull/2784/files as well - this may make sense as a third item on the list of MCP "client"s.

[Artui]

@DouweM I don't think server_id and tool_name fit into args because of how streaming of built-in tools and their args is built. I have now added tool_call_metadata and tool_return_metadata to classes, and I think they now fit the use case better and handle serialization and deserialisation too