feat: Add Claude Opus 4.8 CUA support (#2170)

## Summary
- Add `anthropic/claude-opus-4-8` to the CUA model whitelist.
- Route bare `claude-opus-4-8` model names to the Anthropic CUA client.
- Treat Claude Opus 4.8 as a Claude 4.6+ adaptive-thinking CUA model and
document it in the supported CUA model lists.

## Validation
- `pnpm --filter @browserbasehq/stagehand run typecheck`
- `pnpm run test:core --
packages/core/dist/esm/tests/unit/public-api/llm-and-agents.test.js
packages/core/dist/esm/tests/unit/anthropic-cua-adaptive-thinking.test.js`

Note: the focused test command passed 38 tests, then the test wrapper
printed a non-fatal CTRF conversion warning from `glob/minimatch` under
Node 22.