Better errorhandling and edge cases support.

[LarsArtmann]

I my be a power user, but I am running into ALL sorts of small bugs and edges ALL the time.
I think we could improve this easily by adding more of the https://golangci-lint.run/ linters as default enabled.

I noticed quite some bugs are related to MultiModel or model switching, for example:

1. The cost calculations do not respect model switching (I think)
2. Just ran into this:

I used Google Gemini 3 Pro (Preview) to fix a bug based on a Image and a small prompt.
Or at least find the root cause.
For Costs reasons I wanted to switch to MiniMax M2.1, but... you can see the error.

Expected behavior: Deal with it like we would normally deal with images when the Model does not support them. Or just convert them to a File Path in the prompt.

I would like to see: contextcheck, containedctx, andnoctx.
This would most likely have prevented this Issue: #1855 aka these 2 PRs: #1906 & #1855

This was handwritten in a rage. So sorry for my english, but I love this project and I want to see the quality and usablity go up!