test(env): isolate os.environ in env_loader test so it doesn't leak keys into later tests

tests/test_env_loader.py

@@ -12,9 +12,9 @@ def test_loads_keys_without_overriding_existing(tmp_path, monkeypatch):
         "export ANTHROPIC_API_KEY=exported_style  # inline note\n"
         "ALREADY_SET=should_not_win\n"
     )
-    monkeypatch.setenv("ALREADY_SET", "real_env")     # real env must win
-    for k in ("OLLAMA_API_KEY", "OPENAI_API_KEY", "ANTHROPIC_API_KEY"):
-        monkeypatch.delenv(k, raising=False)          # ensure .env values can land
+    # Isolate os.environ to a throwaway dict so load_dotenv (which mutates the
+    # global environment) can't leak provider keys into later tests.
+    monkeypatch.setattr(os, "environ", {"ALREADY_SET": "real_env"})
 
     loaded = load_dotenv(start=tmp_path)
     assert loaded == str(tmp_path / ".env")