fix(provider): preserve anthropic provider-executed tool pairs

[nexxeln]

summary

• Keep Anthropic's client tool-call normalization, but classify only non-provider-executed tool calls/results as movable client tool parts.
• Preserve provider-executed tool-call/tool-result pairs in assistant content because the AI SDK intentionally represents server-side tools that way.
• Add regressions for client co-located results, provider-executed trailing text, and mixed client/provider-executed tool content.

why this over #25775

• fix(provider): keep tool-call and tool-result paired in anthropic normalize #25775 treats every tool-call and tool-result as one generic tool bucket, which fixes the reported split but also rewrites valid provider-executed server-tool transcripts.
• This keeps the same protection for the reported bug while avoiding unnecessary reordering of Anthropic code execution/web search style provider-executed results.

testing

• bun test test/provider/transform.test.ts
• bun typecheck
• push hook: bun turbo typecheck

Closes #25774

[bbartels]

While you are looking into anthropic issues, would highly appreciate looking into: #14332 as well.

Have a pr out here: #23755, (happy to fix the merge conflicts, but also happy for you to just use this as inspiration)

Your PR, plus a fix for: #14332 would solve all the anthropic related errors we have been running into :)