Issue for this PR

Closes #34544

Type of change

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

What does this PR do?

Variants in a model's config can now carry a limit override, so the same model can have e.g. a 200k-context preset next to its full 1M default:

"glm-5.2": {
  "limit": { "context": 1000000, "output": 131072 },
  "variants": { "200k": { "limit": { "context": 200000 } } }
}

Previously a variant's limit did nothing useful: maxOutputTokens and the compaction check always read the base model.limit, and the raw limit object was merged into provider request options along with the rest of the variant (so it could end up in the request body).

The fix is a small ProviderTransform.withVariantLimit(model, variantID) helper that returns the model with the active variant's limit merged over the base (partial — override just context if that's all you set). It's applied once where the session resolves its model in prompt.ts, so everything downstream (max output tokens via LLMRequestPrep.prepare, the overflow/compaction trigger in overflow.ts) follows the active variant without any signature changes. prepare also applies it itself so direct callers get the same behavior, and now strips limit from the variant before merging request options. The variant limit field is typed in the v1 config schema so it shows up in the config JSON schema (SDK types regenerated).

Not covered here: the context-% display in the TUI/web sidebar still reads the base model limit (it resolves models from the provider list, not the session), and the v2 runner has the same gap — happy to follow up on either if you want.

How did you verify your code works?

• New test in packages/opencode/test/session/llm.test.ts: a config-defined variant with limit: { output: 4096 } produces max_tokens: 4096 in the outgoing request, and no limit key leaks into the request body.
• bun test test/session/llm.test.ts (27 pass), test/provider (421 pass), and prompt/compaction/processor suites (123 pass) in packages/opencode.
• bun typecheck clean in packages/core; packages/opencode shows only the pre-existing src/bus/global.ts error that's also present on a clean dev checkout.

Screenshots / recordings

N/A — no UI change.

Checklist

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