[Bug] Codex Desktop hard-fails with "model at capacity" instead of auto-falling back like Codex Web #44550

Describe the bug
During peak load, Codex Desktop returns "Selected model is at capacity.
Please try a different model." and hard-fails. Codex Web, on the same
account and model, automatically falls back to a less-loaded model and
continues working. This leaves Pro subscribers unable to work in Desktop
whenever the newest models are overloaded.

Environment

• Plan: ChatGPT Pro
• Client: Codex Desktop (ChatGPT.app, v26.903.71938, macOS 26.5)
• Models affected: gpt-6-astra, gpt-5.6-sol, gpt-5.6-terra (consistently);
  gpt-5.5 (intermittent). gpt-5.3-codex-spark works.

To Reproduce

1. In Codex Desktop, select a high-demand model (e.g. gpt-6-astra).
2. Send a message during peak load.
3. Desktop returns "Selected model is at capacity" and stops.
4. Send the same message on Codex Web with the same model — Web either
   succeeds or silently falls back to a working model.

Expected behavior
Codex Desktop should fall back to an available model (as Codex Web does),
or otherwise retry, rather than hard-failing with "at capacity".

Actual behavior
Codex Desktop hard-fails with "Selected model is at capacity" and does
not recover, interrupting long-running tasks.

Frequency
Every day since Sep 7, 2026, during peak hours.

Additional context
Related: #41790, and the "GPT-5.6 Sol repeatedly hits 'Selected model is
at capacity' in Codex Desktop" thread on the OpenAI developer community.