feat(acp): Separate config options for model and variant

[AlvaroParker]

Issue for this PR

Closes #21556 #16543

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

This PR separates the config options to choose model and variant. Previously ACP would send a configOptions for both the model and the variant in the form of provider/model (variant). This changes separate the model and variant into two different configOption and now respect the default model and variant specified in the configuration file.

How did you verify your code works?

Made tests and manual verification via Zed ACP client.

Screenshots / recordings

If this is a UI change, please include a screenshot or recording.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my search results, I found one potentially related PR:

Related PR:

• fix(acp): sync model and variant when activating session modes #18663 - fix(acp): sync model and variant when activating session modes

This PR deals with model and variant synchronization in ACP, which is related to the current PR's focus on separating config options for model and variant. However, this appears to be an older fix addressing synchronization rather than the new feature of separating config options.

The other search results that appeared were either the current PR itself (#21558) or unrelated PRs about different ACP features (thought level config, reasoning level switcher).

No significant duplicate PRs found. The current PR (#21558) appears to be the primary implementation for this feature of separating config options for model and variant.