feat(config): add actionable startup diagnostics and WebUI recovery (#5110)

tests/config/test_env_interpolation.py

@@ -2,6 +2,7 @@ import json
 
 import pytest
 
+from nanobot.config.errors import ConfigLoadError
 from nanobot.config.loader import (
     _resolve_env_vars,
     load_config,
@@ -66,6 +67,22 @@ class TestResolveConfig:
         resolved = resolve_config_env_vars(raw)
         assert resolved.providers.groq.api_key == "resolved-key"
 
+    def test_missing_env_var_reports_config_field(self, tmp_path, monkeypatch):
+        name = "NANOBOT_TEST_MISSING_PROVIDER_KEY"
+        monkeypatch.delenv(name, raising=False)
+        config_path = tmp_path / "config.json"
+        config = Config.model_validate(
+            {"providers": {"openrouter": {"apiKey": f"${{{name}}}"}}}
+        )
+
+        with pytest.raises(ConfigLoadError) as exc_info:
+            resolve_config_env_vars(config, config_path=config_path)
+
+        error = exc_info.value
+        assert error.kind == "missing_env"
+        assert "providers.openrouter.apiKey" in str(error)
+        assert name in str(error)
+
     def test_save_preserves_templates(self, tmp_path, monkeypatch):
         monkeypatch.setenv("MY_TOKEN", "real-token")
         config_path = tmp_path / "config.json"