fix(acp): don't fail session creation when model listing is unavailable

[idosavion]

Summary

Fixes #7427

build_model_state treats model listing as a hard gate on session creation — if the GET to the models endpoint fails, on_new_session returns an error and the session never starts. This breaks proxy-based and deployment-specific providers that only implement the chat completions endpoint (no /models route).

Now build_model_state catches the error, logs a warning, and returns an empty model list. The session starts normally; model selection in the UI just won't be available. This matches providers like Azure and xAI that already return empty model lists through the default fetch_supported_models impl.

Type of Change

• Bug fix
• Tests

AI Assistance

• This PR was created or reviewed with AI assistance

Testing

• test_build_model_state::fetch_error_falls_back_to_current_model_only — previously asserted Err(_), now asserts the function returns the current model with an empty available-models list
• cargo fmt, cargo clippy -D warnings clean

Related Issues

Follow-up to #7112 (model selection support). That PR added build_model_state but made it fatal, so any provider without a models endpoint can't start sessions.

[Copilot]

Pull request overview

This PR fixes a bug where ACP session creation fails silently when using proxy-based providers that don't implement the OpenAI-compatible /models endpoint. The fix changes build_model_state to gracefully degrade by logging a warning and returning an empty model list instead of propagating the error, allowing sessions to start normally with model selection unavailable.

Changes:

• Modified build_model_state to handle fetch_recommended_models errors gracefully instead of propagating them
• Updated function signature to return SessionModelState directly instead of Result<SessionModelState, sacp::Error>
• Updated test case from "fetch error propagates" to "fetch error falls back to current model only" with correct assertions

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

[idosavion]

For sanity, built the custom binary and verified the log is emitted as expected and session is starting:

{
  "level": "WARN",
  "message": "failed to fetch models, model selection will be unavailable",
  "error": "Request failed: Resource not found (404): ",
  "target": "goose_acp::server"
}

[idosavion]

@katzdave - thanks for the quick review!
Anything else I need to do in order to get this merged and deployed as part of the next path/minor version?

[katzdave]

@katzdave - thanks for the quick review! Anything else I need to do in order to get this merged and deployed as part of the next path/minor version?

Nope sorry let's merge!