feat(llm): add GPT-6 Astra, Gemini 3.8 Flash and the current Claude models

- OpenAI: gpt-6-astra
- Google: gemini-3.8-flash and gemini-3.5-flash-lite
- Anthropic: claude-opus-5 and claude-fable-5-1
- models taken out of the picker stay valid through LEGACY_MODELS, so configs
  that name them run without an unknown-model warning

tests/test_model_validation.py

@@ -53,3 +53,14 @@ class ModelValidationTests(unittest.TestCase):
                     client.get_llm()
 
                 self.assertEqual(caught, [])
+
+
+def test_legacy_ids_stay_valid_without_being_offered():
+    from tradingagents.llm_clients.model_catalog import LEGACY_MODELS, MODEL_OPTIONS
+    from tradingagents.llm_clients.validators import validate_model
+
+    for provider, ids in LEGACY_MODELS.items():
+        offered = {v for opts in MODEL_OPTIONS[provider].values() for _, v in opts}
+        for model in ids:
+            assert validate_model(provider, model), model
+            assert model not in offered, f"{model} is legacy but still in the picker"