chore(models): retire deprecated models, simplify thinking config

Trim each provider to current-generation models and drop the special-casing
they required:

- OpenAI: remove gpt-4.1 (deprecated; the only non-reasoning model).
- Anthropic: remove Claude Sonnet 4.5 (legacy; the only Sonnet that 400s on effort).
- Google: remove the Gemini 2.5 line (superseded by 3.x).
- Gemini client: drop the integer thinking_budget mapping; 3.x takes the string
  thinking_level directly.

Effort/reasoning gates stay as defense in depth for custom model IDs. All kept
IDs verified against live APIs.

tests/test_google_api_key.py

@@ -21,7 +21,7 @@ class TestGoogleApiKeyStandardization(unittest.TestCase):
         for msg, kwargs, expected_key in test_cases:
             with self.subTest(msg=msg):
                 mock_chat.reset_mock()
-                client = GoogleClient("gemini-2.5-flash", **kwargs)
+                client = GoogleClient("gemini-3.5-flash", **kwargs)
                 client.get_llm()
                 call_kwargs = mock_chat.call_args[1]
                 self.assertEqual(call_kwargs.get("google_api_key"), expected_key)

tests/test_google_thinking_level.py

@@ -0,0 +1,46 @@
+"""Gemini thinking_level forwarding (Gemini 3.x).
+
+The catalog is Gemini 3.x only, which takes the string ``thinking_level``
+directly. Pro accepts low/high; Flash also accepts minimal/medium — an
+unsupported "minimal" on Pro is mapped to "low".
+"""
+
+from unittest import mock
+
+import pytest
+
+from tradingagents.llm_clients.google_client import GoogleClient
+
+
+def _captured_kwargs(model, **kwargs):
+    captured = {}
+    with mock.patch.object(
+        __import__("tradingagents.llm_clients.google_client", fromlist=["x"]),
+        "NormalizedChatGoogleGenerativeAI",
+        lambda **kw: captured.setdefault("kw", kw),
+    ):
+        GoogleClient(model, api_key="x", **kwargs).get_llm()
+    return captured["kw"]
+
+
+@pytest.mark.parametrize("level", ["minimal", "low", "medium", "high"])
+def test_flash_passes_thinking_level_through(level):
+    kw = _captured_kwargs("gemini-3.5-flash", thinking_level=level)
+    assert kw["thinking_level"] == level
+    assert "thinking_budget" not in kw  # the 2.5-era param is gone
+
+
+def test_pro_remaps_minimal_to_low():
+    kw = _captured_kwargs("gemini-3.1-pro-preview", thinking_level="minimal")
+    assert kw["thinking_level"] == "low"  # Pro doesn't accept "minimal"
+
+
+def test_pro_keeps_high():
+    kw = _captured_kwargs("gemini-3.1-pro-preview", thinking_level="high")
+    assert kw["thinking_level"] == "high"
+
+
+def test_no_thinking_level_is_omitted():
+    kw = _captured_kwargs("gemini-3.5-flash")
+    assert "thinking_level" not in kw
+    assert "thinking_budget" not in kw