[Feature] Optional per-credential Codex client identity isolation #5208

Feature request

Please consider adding an optional mode where each Codex OAuth credential managed by one CLIProxyAPI instance has its own stable client identity.

The goal is predictable isolation for multi-account deployments. This is not a claim about quota enforcement or a request to bypass upstream controls.

Expected user-facing behavior:

• Identity is scoped to the selected OAuth credential.
• It remains stable across restarts and token refreshes.
• It is consistent across HTTP, SSE, and WebSocket requests.
• Existing round-robin, session-affinity, and pinned-auth behavior remains unchanged.
• The feature is optional and disabled by default.

Related: #5027. I understand that issue was closed because there was no evidence connecting identifiers to quota restrictions. This request is simply for optional per-credential isolation as a product capability.