fix(executor): handle CommandCode in-stream errors for combo and account fallback

[berthojoris]

Summary

• Add early inspection (inspectAndWrapCommandCodeResponse) to CommandCode streaming responses to detect in-stream errors before streaming tokens downstream.
• Map in-stream CommandCode errors (e.g. overloaded / 503, rate-limited / 429) to proper non-200 HTTP responses.
• Enables combo and account fallback mechanisms to properly trigger when upstream CommandCode returns an error payload inside an HTTP 200 stream.
• Replays buffered chunks losslessly for normal non-error streams into OpenAI SSE format.
• Add parseError implementation to CommandCodeExecutor and helper parseCommandCodeError.

Test Plan

• tests/unit/commandcode-to-openai.test.js (9 tests passed)
• tests/unit/commandcode-executor.test.js (7 tests passed)
• Baseline verifications passed:
  • verify-alias.mjs
  • verify-oauth-urls.mjs
  • verify-providers.mjs