feat: bump AI SDK to v6 + add Opus 4.6 adaptive thinking

[cascadingstyletrees]

Fixes#13227

Bumps the full AI SDK ecosystem from v5 to v6 and adds Claude Opus 4.6 adaptive thinking support.

Changes

SDK bump (17 files, 199+/171-):

• ai core: 5.0.124 → 6.0.81
• @ai-sdk/provider: 2.0.1 → 3.0.8
• @ai-sdk/provider-utils: 3.0.20 → 4.0.14
• All 16 @ai-sdk/* provider packages to latest

Breaking change fixes:

• Renamed createProviderDefinedToolFactory* → createProviderToolFactory* (6 files)
• Removed deprecated name property from tool factories (6 files)
• LanguageModelV2 → LanguageModel union for V2/V3 compat
• convertToModelMessages is now async → toModelMessages made async + callers updated
• LanguageModelV3Middleware requires specificationVersion
• PromiseLike doesn't have .catch() → wrapped with Promise.resolve()
• Provider factory type relaxed for v3 interface compat

Opus 4.6 variants (transform.ts):

• high: adaptive thinking + effort high
• max: adaptive thinking + effort max
• fast: adaptive thinking + effort high + speed fast

Verification

• bun install ✓
• bun run typecheck ✓ (all 12 packages)
• bun run build --single ✓
• Binary runs, opencode models lists all providers correctly
• Pre-push hook passes (full turbo typecheck pipeline)

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on my search, I found 2 potentially related PRs:

1. PR fix(provider): split Bedrock Opus 4.6 into 200K and 1M variants #13200: fix(provider): split Bedrock Opus 4.6 into 200K and 1M variants

   • Related because it also addresses Opus 4.6 variants, though focused on Bedrock provider

2. PR feat(provider): add adaptive thinking and 1M context support for Claude Opus 4.6 #12342: feat(provider): add adaptive thinking and 1M context support for Claude Opus 4.6

   • Related because it previously added adaptive thinking support for Opus 4.6 - the current PR (13228) may supersede or build upon this earlier work

These could be checking if there's overlap in the Opus 4.6 adaptive thinking implementation between these PRs.

[cascadingstyletrees]

Closing in favor of #12342 which covers the same SDK v6 bump + Opus 4.6 adaptive thinking support, plus additional fixes (token counting, plugin). Apologies for the duplicate.