feat(config): TRADINGAGENTS_* env-var overlay for DEFAULT_CONFIG

Adds a single _ENV_OVERRIDES table in default_config.py with type-aware
coercion (str/int/bool), so users can switch llm_provider, deep/quick
models, backend URL, output language, debate rounds, and the checkpoint
flag purely via .env. Centralizes load_dotenv in the package __init__
so the overlay applies for every entry point (CLI, main.py, programmatic).
Drops the hardcoded model assignments and duplicate dotenv loads in
main.py and cli/main.py. Verified live with OpenAI and Gemini.

#602

tests/test_env_overrides.py

@@ -0,0 +1,98 @@
+"""Tests for TRADINGAGENTS_* env-var overlay onto DEFAULT_CONFIG."""
+
+from __future__ import annotations
+
+import importlib
+
+import pytest
+
+import tradingagents.default_config as default_config_module
+
+
+def _reload_with_env(monkeypatch, **overrides):
+    """Set/clear env vars then reload default_config to re-evaluate DEFAULT_CONFIG."""
+    for key in list(default_config_module._ENV_OVERRIDES):
+        monkeypatch.delenv(key, raising=False)
+    for key, val in overrides.items():
+        monkeypatch.setenv(key, val)
+    return importlib.reload(default_config_module)
+
+
+def test_no_env_uses_built_in_defaults(monkeypatch):
+    dc = _reload_with_env(monkeypatch)
+    assert dc.DEFAULT_CONFIG["llm_provider"] == "openai"
+    assert dc.DEFAULT_CONFIG["deep_think_llm"] == "gpt-5.4"
+    assert dc.DEFAULT_CONFIG["quick_think_llm"] == "gpt-5.4-mini"
+    assert dc.DEFAULT_CONFIG["backend_url"] is None
+    assert dc.DEFAULT_CONFIG["max_debate_rounds"] == 1
+    assert dc.DEFAULT_CONFIG["checkpoint_enabled"] is False
+
+
+def test_string_overrides(monkeypatch):
+    dc = _reload_with_env(
+        monkeypatch,
+        TRADINGAGENTS_LLM_PROVIDER="google",
+        TRADINGAGENTS_DEEP_THINK_LLM="gemini-3-pro-preview",
+        TRADINGAGENTS_QUICK_THINK_LLM="gemini-3-flash-preview",
+        TRADINGAGENTS_LLM_BACKEND_URL="https://example.invalid/v1",
+        TRADINGAGENTS_OUTPUT_LANGUAGE="Chinese",
+    )
+    assert dc.DEFAULT_CONFIG["llm_provider"] == "google"
+    assert dc.DEFAULT_CONFIG["deep_think_llm"] == "gemini-3-pro-preview"
+    assert dc.DEFAULT_CONFIG["quick_think_llm"] == "gemini-3-flash-preview"
+    assert dc.DEFAULT_CONFIG["backend_url"] == "https://example.invalid/v1"
+    assert dc.DEFAULT_CONFIG["output_language"] == "Chinese"
+
+
+def test_int_coercion(monkeypatch):
+    dc = _reload_with_env(
+        monkeypatch,
+        TRADINGAGENTS_MAX_DEBATE_ROUNDS="3",
+        TRADINGAGENTS_MAX_RISK_ROUNDS="2",
+    )
+    assert dc.DEFAULT_CONFIG["max_debate_rounds"] == 3
+    assert isinstance(dc.DEFAULT_CONFIG["max_debate_rounds"], int)
+    assert dc.DEFAULT_CONFIG["max_risk_discuss_rounds"] == 2
+    assert isinstance(dc.DEFAULT_CONFIG["max_risk_discuss_rounds"], int)
+
+
+@pytest.mark.parametrize(
+    "raw,expected",
+    [
+        ("true", True), ("True", True), ("1", True), ("yes", True), ("on", True),
+        ("false", False), ("False", False), ("0", False), ("no", False), ("off", False),
+    ],
+)
+def test_bool_coercion(monkeypatch, raw, expected):
+    dc = _reload_with_env(monkeypatch, TRADINGAGENTS_CHECKPOINT_ENABLED=raw)
+    assert dc.DEFAULT_CONFIG["checkpoint_enabled"] is expected
+
+
+def test_empty_env_value_is_passthrough(monkeypatch):
+    """Empty TRADINGAGENTS_* values must not clobber the built-in default."""
+    dc = _reload_with_env(
+        monkeypatch,
+        TRADINGAGENTS_LLM_PROVIDER="",
+        TRADINGAGENTS_MAX_DEBATE_ROUNDS="",
+    )
+    assert dc.DEFAULT_CONFIG["llm_provider"] == "openai"
+    assert dc.DEFAULT_CONFIG["max_debate_rounds"] == 1
+
+
+def test_invalid_int_raises(monkeypatch):
+    """Garbage int values should surface a ValueError at import, not silently misconfigure."""
+    monkeypatch.setenv("TRADINGAGENTS_MAX_DEBATE_ROUNDS", "not-a-number")
+    with pytest.raises(ValueError):
+        importlib.reload(default_config_module)
+    # Restore module state for subsequent tests in this process
+    monkeypatch.delenv("TRADINGAGENTS_MAX_DEBATE_ROUNDS", raising=False)
+    importlib.reload(default_config_module)
+
+
+def test_unknown_env_var_is_ignored(monkeypatch):
+    """Env vars outside _ENV_OVERRIDES must not bleed into DEFAULT_CONFIG."""
+    dc = _reload_with_env(
+        monkeypatch,
+        TRADINGAGENTS_NONEXISTENT_KEY="oops",
+    )
+    assert "nonexistent_key" not in dc.DEFAULT_CONFIG