[Bug]: Anthropic OAuth tokens stopped working - "OAuth authentication is currently not supported"

Describe the bug

All Anthropic OAuth tokens (sk-ant-oat01-*) obtained via claude setup-token and configured through OpenClaw are now returning 401 authentication errors from Anthropic API.

Steps to reproduce

1. Configure Anthropic auth using openclaw configure --section model
2. Choose "Anthropic token (paste setup-token)"
3. Run claude setup-token and paste the generated OAuth token
4. Try to use any Anthropic model (opus-4-5, sonnet-4-5, etc.)
5. Observe 404/401 errors

Expected behavior

OAuth tokens should authenticate successfully with Anthropic Messages API.

Actual behavior

API returns:

{"type":"error","error":{"type":"authentication_error","message":"OAuth authentication is currently not supported."}}

Direct API test

curl -s https://api.anthropic.com/v1/messages \
  -H "Authorization: Bearer sk-ant-oat01-..." \
  -H "anthropic-version: 2023-06-01" \
  -H "Content-Type: application/json" \
  -d '{"model":"claude-opus-4-5-20250918","max_tokens":50,"messages":[{"role":"user","content":"test"}]}'

Returns: "OAuth authentication is currently not supported."

Environment

• OpenClaw version: 2026.2.4 (commit 821520a, also tested on 4629054)
• Platform: macOS Darwin 25.2.0
• Install method: git clone + pnpm build
• Auth profiles tested: claude-max (subscription), iOS sub - all OAuth tokens fail

Timeline

This was working correctly until ~4 hours ago (around when opus-4.6 commit eb80b9a was merged). May be related to Anthropic API changes, not OpenClaw code.

Workaround

Use standard Anthropic API keys from console.anthropic.com instead of OAuth tokens (per-token billing instead of subscription).

Additional context

• Tested with multiple fresh OAuth tokens - all fail the same way
• OpenAI Codex OAuth tokens also have permission issues (missing api.model.read scope)
• No related issues found in recent GitHub issues
• Gateway logs show HTTP 404 from Anthropic API endpoints

Related issues

This may affect all users using OAuth-based authentication (Claude Max, iOS subscriptions, ChatGPT Plus) rather than API keys.