codex models fail with unsupported_api_for_model when model list fetch is rate-limited

[chcaru]

Describe the bug

When multiple concurrent copilot processes use gpt-5.3-codex, they fail with unsupported_api_for_model because the CLI sends requests to /chat/completions instead of /responses.

The issue occurs when the listModels API call is rate-limited (HTTP 429). A single CLI instance works fine because the model list loads successfully.

Root cause (from inspecting the 0.0.415 bundle):

The responses API routing in KXe.getCompletionWithTools requires both conditions to be true:

U7e(settings, clientOptions) && model?.supported_endpoints?.includes("/responses")

• U7e() checks clientOptions.thinkingMode || featureFlag("copilot_swe_agent_enable_responses_api")
• model comes from chatClient.modelPromise — a model list fetch from the API

When the model list fetch fails (HTTP 429), model is null, so supported_endpoints?.includes("/responses") is falsy. The CLI falls back to /chat/completions, which rejects codex models. It then retries the same failing endpoint 5 times without re-attempting the model list fetch.

Affected version

0.0.415

Steps to reproduce the behavior

1. Run multiple concurrent copilot -p processes with --model gpt-5.3-codex
2. The listModels endpoint gets rate-limited (429)
3. All instances fall back to /chat/completions and fail with:

   {"error":{"message":"model \"gpt-5.3-codex\" is not accessible via the /chat/completions endpoint","code":"unsupported_api_for_model"}}

Expected behavior

When the model list is unavailable, the CLI should either:

• Handle the unsupported_api_for_model error by retrying with the /responses endpoint
• Retry the model list fetch with backoff before giving up
• Treat thinkingMode alone (without model metadata) as sufficient to route to /responses

Additional context

From the process logs:

[ERROR] Error loading models: Error: Failed to list models: 429

Followed by 6 consecutive failures on /chat/completions with unsupported_api_for_model, then exit code 1.