Automatically reduce LLM tools to stay below max threshold

[digitarald]

Agent mode needs to intelligently reduce the number of LLM-included tools from what is currently enabled in the tool picker, to avoid failing chat requests or forcing users to micro-manage the enabled MCP servers and tools (which isn't an easy task in the tool picker, which many users are not even aware of).

This is important as user reports for the max 128 tools errors have been steadily increasing and we are making it even easier to install new MCP servers.

There are a few ideas for how solve it, but it needs to be fast and accurate enough to be seamless.

cc @aeschli @connor4312

[connor4312]

Merging into #248021