fix(opencode): reject unknown --variant instead of silently dropping it

[C0d3N1nja97342]

Issue for this PR

Closes #40182

Type of change

• Bug fix

What does this PR do?

opencode run --variant <value> accepted any string. When the value was not among the model's declared variants, the LLM request prep did a bracket lookup model.variants[variant] that returned undefined and merged nothing - so the run proceeded at the model's default effort level while the session record claimed an effort level that was never applied. Exit was 0 with no warning or log line. The reporter captured the outgoing HTTP request body to confirm reasoning_effort was absent for unknown values.

Why withVariant didn't catch this: packages/core/src/session/runner/model.ts already returns VariantUnavailableError for unknown variants - but the opencode run path uses its own LLM service (packages/opencode/src/session/llm/request.ts:80-83) which never calls withVariant. It does the bracket lookup directly, so the core guard does not cover opencode run. The two paths also use different representations of variants (array vs record).

Fix: validate an explicit --variant against the model's declared variants in createUserMessage (packages/opencode/src/session/prompt.ts) before it is recorded on the session. When a variant is supplied, fetch the full catalog model and throw a NamedError listing the valid variants if the value is unknown. This makes opencode run --variant <typo> exit non-zero with an actionable message, and stops the unapplied value from being persisted to session metadata.

"default" is passed through unchecked (it is resolved later to model.request.variant), matching the existing semantics.

How did you verify your code works?

Added a regression test in packages/opencode/test/session/prompt.test.ts (mirroring the existing "applies agent variant" and "unknown agent throws typed error" tests): it supplies variant: "totally-invalid-xyz" against a model declaring variants: { xhigh: {}, high: {} } and asserts the prompt fails with a NamedError.Unknown whose message contains Unknown variant "totally-invalid-xyz", the test/test-model identifier, and the valid values (high, xhigh).

Caveat: I could not run the packages/opencode test suite locally - the package depends on @opentui/solid, whose transitive deps (@babel/core/gensync) fail to resolve under Bun on Windows + Docker volume mounts (symlink/EACCES issues). The test file and the change both transpile cleanly (bun build --no-bundle), and the test follows the exact assertion pattern of the existing variant/agent-error tests in the same file. I'd appreciate a maintainer running bun test test/session/prompt.test.ts in CI to confirm.

Checklist

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

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