fix(tui): cycle model variants from default

[tobwen]

Issue for this PR

Closes #36095

Type of change

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

What does this PR do?

This PR fixes TUI variant cycling when a model defines a real variant named default. The cycle logic now uses a non-default cycle order internally, so ctrl+t moves through configured variants such as high, max, and none before returning to the default state.

The previous logic used the full variant list. Because set(undefined) stores default, a list like high, max, default, and none made the cycle jump from default to none, then back to default. The visible variant list and dialog behavior are unchanged.

How did you verify your code works?

• Added regression coverage for nextVariantInCycle with high, max, default, and none.
• Verified the regression test fails with the old cycle behavior: default returned none instead of high.
• Ran bun test test/context/local.test.ts from packages/tui.
• Ran bun typecheck from packages/tui.
• Built the single Linux binary with bun run build -- --single from packages/opencode and confirmed the build smoke test passed.
• Manually verified that ctrl+t cycles through default, high, max, and none for the affected model.

Screenshots / recordings

Not included. The change affects keyboard cycling behavior in the TUI.

Checklist

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