Add support for AI SDK 7

[OskarLebuda]

Resolves #597

Migrate to AI SDK 7 https://vercel.com/blog/ai-sdk-7

Bumps ai-sdk to v7 and @ai-sdk/provider to v4 across the repo (provider, examples, demos).

Breaking: peer deps now require ai@^7 / @ai-sdk/provider@^4. AI SDK 6 users should stay on the previous major.

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[FradSer]

We depend on workers-ai-provider with ai + @ai-sdk/react in production and are blocked from upgrading to AI SDK v7 until this lands. Would appreciate a review/merge when ready — happy to help test a prerelease.