fix(opencode): handle removed OpenAI OAuth auth

[opencode-agent]

Summary

• read the current OpenAI auth state before applying OAuth-only request mutations
• pass requests through unchanged when OAuth auth was removed or replaced after provider loading
• cover the mid-session auth removal race with a regression test

Root cause

The OpenAI Codex fetch wrapper outlived the OAuth credential that created it. When the credential was removed while a session was active, getAuth() returned undefined and the wrapper dereferenced .type, producing the minified undefined is not an object (evaluating 'M.type') stream failure.

Verification

• Exact headless repro against the 1.18.11 release binary: reproduced the reported M.type failure by removing OAuth auth after provider loading
• bun test test/plugin/codex.test.ts --timeout 30000 (18 passed)
• git diff --check

Requested by: @Hona (Hona via Slack)