Add `OPENAI_API_KEY` support in `gaia_solving_agent` and update `.example.env` with example key.

.example.env

@@ -11,6 +11,7 @@
 ###########################################################
 
 HF_TOKEN = "hf_xxxxxx"
+OPENAI_API_KEY = "sk-proj-xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx-xxxxxxxxxxxxxxxxxxx-x-xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx-xxxxxxxxxxxxxxxxxxxx-xx-xxxxx"
 NEBIUS_API_TOKEN = "xxxxxxxx"
 MISTRAL_API_KEY = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
 # See https://docs.llamaindex.ai/en/stable/module_guides/observability/#llamatrace-hosted-arize-phoenix

src/gaia_solving_agent/__init__.py

@@ -4,6 +4,7 @@ from dotenv import load_dotenv
 # Load the .env file
 load_dotenv()
 HF_API_TOKEN = os.getenv("HF_TOKEN")
+OPENAI_API_KEY = os.getenv("OPENAI_API_KEY")
 NEBIUS_API_KEY = os.getenv("NEBIUS_API_TOKEN")
 MISTRAL_API_KEY = os.getenv("MISTRAL_API_KEY")
 PHOENIX_API_KEY = os.getenv("PHOENIX_API_KEY")