chore: upgrade ai sdk to v6

[uinstinct]

Description

Upgrade aisdk along with corresponding adapters to v6. Also upgrade the functions in packages/openai-adapters to use v6 methods.

requires #10622 to be merged first

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Checklist

• [] I've read the contributing guide
• [] The relevant docs, if any, have been updated or created
• [] The relevant tests, if any, have been updated or created

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Tests

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Summary by cubic

Upgraded the adapters to AI SDK v6 and aligned tool, message, and stream handling with the new API to ensure consistent behavior across OpenAI and Anthropic. Removed legacy feature flags and docs; tests now target the unified AI SDK provider.

• Dependencies

  • ai: ^6.0.86
  • @ai-sdk/openai: ^3.0.29
  • @ai-sdk/anthropic: ^3.0.44

• Refactors

  • Use createAnthropic for Anthropics; remove Vercel SDK feature flags and provider stubs.
  • Map max_tokens to maxOutputTokens; keep stop/top_p behavior consistent.
  • Update tool conversion to use inputSchema and ensure top-level object schemas.
  • Convert assistant tool calls to input (was args) and adjust message mapping.
  • Update stream converter to v6 event names and shapes (text-/reasoning-start|delta|end with ids, file/source payloads).
  • Remove outdated VERCEL_AI_SDK.md and legacy tests; add tests for the ai-sdk provider with namespaced models (e.g., openai/gpt-4o-mini).

^{Written for commit 7cca187. Summary will update on new commits.}

[cubic-dev-ai]

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