reproduction: customProvider loses video models from v3 fallback providers

[ai-sdk-factory]

Bug

customProvider loses video models from v3 fallback providers

Reproduction

• The published combination ai@7.0.106, @ai-sdk/fal@2.0.58, and zod@4.4.3 reproduced the issue: direct lookup returned v3, alias and registry lookups returned v4, but custom-provider fallback threw AI_NoSuchModelError.
• packages/ai/src/model/as-provider-v4.ts omits the experimental videoModel method when adapting ProviderV3, while packages/ai/src/registry/custom-provider.ts attempts video fallback lookup on that adapted provider.
• Upgrading to the checked-out @ai-sdk/fal@3.0.45 avoids the failure, but ProviderV3 remains an explicitly accepted fallback type, so the older provider behavior is supported rather than invalid.
• The reproduction at examples/ai-functions/src/reproduction/custom-provider-v3-video-fallback.ts confirms direct, alias, and registry lookup succeed, then fails because fallback lookup does not return the v3 video model adapted to v4.

Relevant Documentation

• https://ai-sdk.dev/docs/reference/ai-sdk-core/custom-provider
• https://ai-sdk.dev/docs/reference/ai-sdk-core/provider-registry
• https://ai-sdk.dev/docs/ai-sdk-core/provider-management

Related Issues

Reproduces #21110