CLI sends wrong model ID to custom OpenAI-compatible endpoint, killing the session

Bug Report: CLI sends wrong model ID to custom OpenAI-compatible endpoint

Describe the bug

When using a custom OpenAI-compatible endpoint with a non-OpenAI model name (e.g. mimo-v2.5), the CLI sends gpt-5.4-nano as the model in the API request body instead of the configured model name. The custom endpoint rejects the unknown model with HTTP 401, retries exhaust, and the session terminates.

Affected version

1.0.82 (output of copilot --version)

Steps to reproduce the behavior

1. Configure a custom OpenAI-compatible endpoint with a non-OpenAI model name (e.g. mimo-v2.5) as the session model
2. Start or resume a Copilot CLI session targeting that endpoint
3. Send any prompt (e.g. hello)
4. The session fails within seconds

Expected behavior

The CLI should send the configured model name (e.g. mimo-v2.5) in the model field of the API request body, without internal substitution.

Observed behavior

The session event log shows the model field is overridden to gpt-5.4-nano before the API call:

model_call_started → "model":"gpt-5.4-nano"   (expected: "mimo-v2.5")
model_call_failure → HTTP 401, body: "Model gpt-5.4-nano is not supported"
turn_retry         → reason: "transient_auth_error"
session.error      → "Failed to get response from the AI model;
                      retried 5 times (total retry wait time: 31.26 seconds)
                      Last error: 500 Internal server error"

Note: The retry log shows 3 logged retries but the session error reports 5 total — some retries may have occurred before logging began.

Additional context

• The endpoint's GET /v1/models lists mimo-v2.5 as available
• Direct API call with "model":"mimo-v2.5" → HTTP 200 (works)
• Direct API call with "model":"gpt-5.4-nano" → HTTP 401 (model does not exist on this endpoint)
• gpt-5.4-nano is an OpenAI model — it appears the CLI maps unrecognized model names to gpt-5.4-nano before sending, which breaks custom endpoints that don't host that model
• The HTTP 401 comes from the custom endpoint; the final HTTP 500 comes from the CLI's retry layer after all attempts are exhausted
• OS: Windows, PowerShell

Start at the CLI entry point and trace how the configured session model is selected and placed into the API request for custom endpoints. Reproduce with a custom endpoint using mimo-v2.5, then verify the request preserves that model and the session completes successfully without substituting gpt-5.4-nano.