fix(cortex-engine): return ProviderNotFound error for unknown providers

Fixes bounty issue #1297

When a user specifies an unknown provider (e.g., --model invalid/model),
the code now returns a clear ProviderNotFound error instead of an
ApiKeyNotFound error which was misleading since the issue is the
unrecognized provider, not missing authentication.

Author: factorydroid

cortex-engine/src/client/mod.rs

@@ -110,20 +110,10 @@ pub fn create_client(
             model.to_string(),
             base_url.map(std::string::ToString::to_string),
         ))),
-        // For any other provider, try OpenAI-compatible API
-        _ => {
-            if api_key.is_empty() {
-                Err(crate::error::CortexError::ApiKeyNotFound {
-                    provider: provider_id.to_string(),
-                })
-            } else {
-                Ok(Box::new(OpenAiClient::new(
-                    model.to_string(),
-                    api_key.to_string(),
-                    base_url.map(std::string::ToString::to_string),
-                )))
-            }
-        }
+        // Unknown provider - return clear error
+        _ => Err(crate::error::CortexError::ProviderNotFound {
+            provider: provider_id.to_string(),
+        }),
     }
 }