bug: gateway providerOptions drops explicit provider namespace under an unrecognized model-ID alias

Description

ProviderTransform.providerOptions() (in packages/opencode/src/provider/transform.ts, the @ai-sdk/gateway branch) buckets every non-gateway top-level provider option under a single key derived from the gateway model-ID prefix (model.api.id.split("/")[0], via SLUG_OVERRIDES).

When that prefix is an internal routing alias rather than a recognized AI SDK provider slug (e.g. a centrally-configured virtual-model-catalog ID that isn't literally "openai" or "anthropic"), there is no way to address a specific upstream provider's providerOptions namespace directly. An explicit, already-correctly-namespaced option such as:

{ "gateway": { "zeroDataRetention": true }, "openai": { "store": false } }

gets folded entirely into the alias slug's bucket instead of surviving as its own top-level providerOptions.openai key — so the AI SDK provider package that actually reads providerOptions.openai never sees it. This breaks any setup that needs OpenAI-specific options (e.g. store: false for zero-data-retention / stateless-reasoning workflows) on a gateway model whose ID isn't already prefixed with a recognized provider slug.

I have a fix and tests ready in a PR: adds a reserved options.providerOptions wrapper as an explicit escape hatch, consumed and stripped before legacy bucketing runs so every other option keeps byte-for-byte identical legacy behavior. See the linked PR for full details and design-alternatives considered.

Plugins

None — reproducible directly against ProviderTransform.providerOptions() with a unit test, no plugins involved.

OpenCode version

Reproduces against dev at commit d6855b6 (opencode-ai 1.18.29), and against the opencode-ai@1.18.15 npm release.

Steps to reproduce

1. Configure a gateway model whose api.id prefix is not one of the AI SDK's recognized provider slugs (e.g. a custom-aliased model ID such as acme/gpt-5.1 routed through @ai-sdk/gateway).
2. Set model options that include an explicit provider namespace, e.g. { gateway: { zeroDataRetention: true }, openai: { store: false } }.
3. Call ProviderTransform.providerOptions(model, options).
4. Observe the result nests openai inside the alias-derived slug (e.g. { gateway: {...}, acme: { openai: { store: false } } }) instead of preserving it as a top-level openai key — so the OpenAI provider package never receives store: false.

Screenshot and/or share link

N/A — this is a pure function bug reproducible with a unit test; see the failing/passing test cases in the linked PR.