fix: correct Groq provider detection test to use model string

- Change test to pass groq/llama3-70b-8192 as first argument to trigger provider detection
- Previously was incorrectly using :mock provider with groq model string

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: azmaveth

test/ex_llm/providers/groq_test.exs

@@ -54,8 +54,7 @@ defmodule ExLLM.Providers.GroqTest do
       messages = [%{role: "user", content: "Hello"}]
 
       {:ok, response} =
-        ExLLM.chat(:mock, messages,
-          model: "groq/llama3-70b-8192",
+        ExLLM.chat("groq/llama3-70b-8192", messages,
           mock_response: "Test response"
         )