* fix(claude): forward client-negotiated thinking-binding-controls and thinking-display-updates betas
Gateways drops anthropic-beta tokens not on FORWARDABLE_CLIENT_BETAS, so
@ai-sdk/anthropic Fable 5.1 requests carrying thinking.block_binding were
rejected upstream with thinking.adaptive.block_binding: Extra inputs are
not permitted even when the client negotiated
thinking-binding-controls-2026-08-01 correctly. Same class for
thinking.display updates (thinking-display-updates-2026-08-18).
Regression guard: tests/unit/thinking-binding-controls-beta-forward.test.ts
* chore(changelog): fragment for #12989
* chore: trim comments