feat: add variant support for commands (#7713)

[koichincom]

What does this PR do?

Fixes #7713 by the following:

• Adds a variant field to the commands configuration.
• Variant needs to be specified with the model (throws an error if not)
• Invalid variant names show a help message and throw an error (no fuzzy search)

How did you verify your code works?

• Added model and variant fields to the existing packages/opencode/test/config/config.test.ts for config parsing.
• Performed three manual tests by creating an opencode.json in the root:
  • Valid: model and variant are specified (used a temporary toast to verify the variant change).
  • Without variant: only the model is specified.
  • Invalid variant: an invalid variant name was used.

Consideration

When #7156 or #7140 lands, its model-default variant feature might need to be integrated.