…atewayProviderOptions (#13855)

## Background

When provider options types were recently renamed from
`{Provider}ProviderOptions` to `{Provider}{ModelType}Options` across all
providers (see #12443), the AI Gateway's type was renamed from
`GatewayProviderOptions` to `GatewayLanguageModelOptions`. However,
unlike other providers, the gateway's options are truly provider-wide
(routing, fallbacks, tags, BYOK, etc.) — not specific to any model type.
The original name was more accurate.

## Summary

- Renamed `GatewayLanguageModelOptions` back to `GatewayProviderOptions`
as the primary export, with `GatewayLanguageModelOptions` kept as a
deprecated alias
- Updated all examples, docs, and type tests to use
`GatewayProviderOptions`
- Added a note to `contributing/providers.md` documenting when
`{Provider}ProviderOptions` should be used instead of
`{Provider}{ModelType}Options`

## Checklist

- [x] Tests have been added / updated (for bug fixes / features)
- [x] Documentation has been added / updated (for bug fixes / features)
- [x] A _patch_ changeset for relevant packages has been added (for bug
fixes / features - run `pnpm changeset` in the project root)
- [x] I have reviewed this pull request (self-review)