fix(core): install AI SDK v6 providers by default

[opencode-agent]

Refs #50960

What

The AI SDK bridge only supports AI SDK v6 providers (the LanguageModelV3 interface). Official @ai-sdk/* packages now publish AI SDK v7 under npm latest and keep the v6 line under the ai-v6 dist-tag, so an unversioned aisdk:@ai-sdk/<name> silently installed a v7 provider.

The v7 providers expect a different prompt format. In #50960, @ai-sdk/openai-compatible@3.x reads part.data.type on a v6-shaped file part, matches no case, and serializes the image as null, which breaks the session on every later turn.

loadSDKFactory now installs an unversioned official package (@ai-sdk/<name> with no version, range, or tag) from @ai-sdk/<name>@ai-v6. Explicit specs, non-@ai-sdk packages, and file:// packages are unchanged.

This doesn't add AI SDK v7 support or any spec-version gate.

Verification

• Every official @ai-sdk/* provider package in the AI SDK repo has an ai-v6 tag that depends on @ai-sdk/provider 3.x.
• I checked every npm package in models.dev, at both provider and model level. Native routes or bundled plugins handle all official @ai-sdk/* packages there, so none reach loadSDKFactory. The rewrite applies only to user-configured aisdk:@ai-sdk/<name> packages that have no native mapping.
• I ran real installs through loadSDKFactory in an isolated cache. @ai-sdk/openai-compatible, @ai-sdk/deepseek, and @ai-sdk/fireworks resolved to their ai-v6 versions, and each reported specificationVersion: "v3". Third-party models.dev packages still install as written.
• bun test test/plugin/provider-dynamic.test.ts and bun typecheck in packages/core pass.

Requested by: @rekram1-node (Aiden Cline via Slack)