fix: remove flaky tag from stable Groq provider detection test

- The test was already synchronous by default (no async: true)
- Removing @tag :flaky since test passes consistently in both isolation and full suite
- Test validates provider/model string parsing (groq/llama3-70b-8192 format)

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

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

Author: azmaveth

test/ex_llm/providers/groq_test.exs

@@ -49,7 +49,6 @@ defmodule ExLLM.Providers.GroqTest do
   end
 
   describe "provider detection" do
-    @tag :flaky
     test "provider/model string works with mock adapter" do
       # Test with mock to avoid real API calls
       messages = [%{role: "user", content: "Hello"}]