fix(ai): guard unsupported Claude Opus 5.5 request options

[opencode-agent]

Summary

• Reject explicit disabled/manual thinking and forced tool choices for Claude Opus 5.5 at the Anthropic Messages request boundary. Anthropic returns HTTP 400 for these shapes; a local InvalidRequest gives the caller a valid alternative without silently discarding a thinking budget or required tool behavior.
• Preserve adaptive thinking, auto/none tool choice, and older Opus request behavior, including gateway-qualified model IDs.

Verification

• bun test test/provider/anthropic-messages.test.ts test/provider/anthropic-thinking-binding.test.ts — 91 passed.
• bun typecheck from packages/ai, bun run lint, Prettier check, and git diff --check passed.
• Root bun run check could not complete on this host: Astro rejects its Node 18 installation; a Bun-as-node retry was terminated under host resource contention.

Anthropic sources: September 22 release notes and Opus 5.5 API changes.

Requested by: @nexxeln (nexxel via Slack)