[action] Adaptive thinking enabled by default; manual extended thinking removed in Claude Sonnet 5

In Claude Sonnet 5, adaptive thinking is now on by default. Manual extended thinking is removed and returns a 400 error. Setting sampling parameters (temperature, top_p, top_k) to non-default values also returns a 400 error. Affected: anthropic/claude-sonnet-5. Severity: action_required. Verification: verified.