fix(models): handle google provider import errors and add dependency (#952)

* fix(models): improve provider import guidance and add google provider dep

* Apply suggestions from code review

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* fix(reflection): prefer provider install hint on transitive import errors

---------

Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

backend/tests/test_reflection_resolvers.py

@@ -0,0 +1,46 @@
+"""Tests for reflection resolvers."""
+
+import pytest
+
+from src.reflection import resolvers
+from src.reflection.resolvers import resolve_variable
+
+
+def test_resolve_variable_reports_install_hint_for_missing_google_provider(monkeypatch: pytest.MonkeyPatch):
+    """Missing google provider should return actionable install guidance."""
+    def fake_import_module(module_path: str):
+        raise ModuleNotFoundError(f"No module named '{module_path}'", name=module_path)
+
+    monkeypatch.setattr(resolvers, "import_module", fake_import_module)
+
+    with pytest.raises(ImportError) as exc_info:
+        resolve_variable("langchain_google_genai:ChatGoogleGenerativeAI")
+
+    message = str(exc_info.value)
+    assert "Could not import module langchain_google_genai" in message
+    assert "uv add langchain-google-genai" in message
+
+
+def test_resolve_variable_reports_install_hint_for_missing_google_transitive_dependency(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """Missing transitive dependency should still return actionable install guidance."""
+
+    def fake_import_module(module_path: str):
+        # Simulate provider module existing but a transitive dependency (e.g. `google`) missing.
+        raise ModuleNotFoundError("No module named 'google'", name="google")
+
+    monkeypatch.setattr(resolvers, "import_module", fake_import_module)
+
+    with pytest.raises(ImportError) as exc_info:
+        resolve_variable("langchain_google_genai:ChatGoogleGenerativeAI")
+
+    message = str(exc_info.value)
+    # Even when a transitive dependency is missing, the hint should still point to the provider package.
+    assert "uv add langchain-google-genai" in message
+def test_resolve_variable_invalid_path_format():
+    """Invalid variable path should fail with format guidance."""
+    with pytest.raises(ImportError) as exc_info:
+        resolve_variable("invalid.variable.path")
+
+    assert "doesn't look like a variable path" in str(exc_info.value)