Add probe_model_for_provider to fix false doctor warning on ChatGPT/Codex backend

doctor and provider_auth used cheapest_model (gpt-5-mini) for connectivity
probes, but gpt-5-mini is rejected by the ChatGPT/Codex backend. Adds
probe_model_for_provider() which returns gpt-5.4-mini for OpenAI and falls
back to the default model for other providers.

Fixes #96

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/crates/fabro-cli/src/doctor.rs

@@ -855,16 +855,9 @@ async fn probe_daytona() -> Option<Result<(), String>> {
     )
 }
 
-pub(crate) fn cheapest_model(provider: Provider) -> String {
-    let models = fabro_llm::catalog::list_models(Some(provider.as_str()));
-    models
-        .iter()
-        .min_by(|a, b| {
-            let cost_a = a.costs.input_cost_per_mtok.unwrap_or(f64::MAX);
-            let cost_b = b.costs.input_cost_per_mtok.unwrap_or(f64::MAX);
-            cost_a.total_cmp(&cost_b)
-        })
-        .map(|m| m.id.clone())
+pub(crate) fn probe_model(provider: Provider) -> String {
+    fabro_llm::catalog::probe_model_for_provider(provider.as_str())
+        .map(|m| m.id)
         .unwrap_or_else(|| format!("unknown-{}", provider.as_str()))
 }
 
@@ -873,7 +866,7 @@ async fn probe_llm_provider(
     provider: Provider,
 ) -> (Provider, Result<(), String>) {
     let request = fabro_llm::types::Request {
-        model: cheapest_model(provider),
+        model: probe_model(provider),
         messages: vec![fabro_llm::types::Message::user("hi")],
         provider: Some(provider.as_str().to_string()),
         tools: None,

lib/crates/fabro-cli/src/provider_auth.rs

@@ -162,7 +162,7 @@ pub(crate) async fn validate_api_key(provider: Provider, api_key: &str) -> Resul
         .await
         .map_err(|e| e.to_string())?;
 
-    let params = fabro_llm::generate::GenerateParams::new(doctor::cheapest_model(provider))
+    let params = fabro_llm::generate::GenerateParams::new(doctor::probe_model(provider))
         .provider(provider.as_str())
         .prompt("Say OK")
         .max_tokens(16)

lib/crates/fabro-llm/src/catalog.rs

@@ -56,6 +56,26 @@ pub fn default_model() -> ModelInfo {
         .expect("catalog.json must contain at least one default model")
 }
 
+/// Get the probe model for a provider — the cheapest model known to work for
+/// connectivity checks.  Falls back to the default model when no explicit
+/// override is configured.
+#[must_use]
+pub fn probe_model_for_provider(provider: &str) -> Option<ModelInfo> {
+    let provider = canonical_provider(provider);
+    // Provider-specific overrides where the cheapest model is unsuitable
+    // (e.g. gpt-5-mini is rejected by the ChatGPT/Codex backend).
+    let override_id: Option<&str> = match provider {
+        "openai" => Some("gpt-5.4-mini"),
+        _ => None,
+    };
+    if let Some(id) = override_id {
+        if let Some(info) = get_model_info(id) {
+            return Some(info);
+        }
+    }
+    default_model_for_provider(provider)
+}
+
 /// List all known models, optionally filtered by provider (Section 2.9).
 #[must_use]
 pub fn list_models(provider: Option<&str>) -> Vec<ModelInfo> {
@@ -608,6 +628,24 @@ mod tests {
         );
     }
 
+    #[test]
+    fn probe_model_openai_returns_override() {
+        let m = probe_model_for_provider("openai").unwrap();
+        assert_eq!(m.id, "gpt-5.4-mini");
+    }
+
+    #[test]
+    fn probe_model_anthropic_returns_default() {
+        let m = probe_model_for_provider("anthropic").unwrap();
+        assert_eq!(m.id, "claude-opus-4-6");
+    }
+
+    #[test]
+    fn probe_model_gemini_returns_default() {
+        let m = probe_model_for_provider("gemini").unwrap();
+        assert_eq!(m.id, "gemini-3.1-pro-preview");
+    }
+
     #[test]
     fn closest_model_opus_to_gemini() {
         let opus = get_model_info("claude-opus-4-6").unwrap();