fix(ai): support Sonnet 5.5 Messages request shapes - #52045

Why

Anthropic's September 28 Sonnet 5.5 launch introduced thinking: { type: "between_tools" }, but @opencode/ai rejected that valid input. Its existing default thinking-binding policy would also append block_binding to this shape, which Sonnet 5.5 rejects. The model additionally rejects forced tool choice, legacy thinking controls, and non-default sampling. These are Sonnet 5.5 native Messages request constraints, distinct from the closed, unmerged Opus 5.5 PR #51302.

Change

• Encode explicit between_tools unchanged without a binding field or binding beta header; retain adaptive thinking's existing binding default.
• Reject incompatible between_tools fields, effort levels, and per-message effort changes, rather than stripping requested controls.
• Give Sonnet 5.5 callers a typed InvalidRequest for forced tool selection, disabled/manual thinking, and disallowed sampling, instead of silently changing intent. Preserve accepted auto/none, temperature 1, and top-p >= 0.99.
• Scope the compatibility checks to Sonnet 5.5 on the native Anthropic Messages protocol, including gateway-qualified IDs. Older models keep their request behavior.

Verification

• Regression tests failed before the implementation and now pass: bun test test/provider/anthropic-messages.test.ts test/provider/anthropic-thinking-binding.test.ts test/effort-updates.test.ts --only-failures — 129 passed.
• bun typecheck from packages/ai, bun run lint, Prettier check, and git diff --check passed.
• Root bun run check is blocked by this host's Node 18: @opencode/posts requires Node >=22.12 for Astro; the affected AI package typecheck passed independently.

Sources: Sonnet 5.5 migration guide, Sonnet 5.5 API changes, and Messages API sampling parameter compatibility.

Requested by: @nexxeln (nexxel via Slack)