Anthropic: add section to docs about structured outputs and tool inputs

[gr2m]

Follow up to #8914

Maybe add a new section to https://ai-sdk.dev/providers/ai-sdk-providers/anthropic#language-models

[dhofheinz]

I've submitted PR #9229 to address this. The new section includes complete code examples for both streamObject and streamText with the anthropic-beta header for incremental streaming.