fix: Import FoundBy from correct module in tests

Fixed Pydantic validation error by importing FoundBy from modules.base
instead of models.finding_schema. The ModuleFinding class in base.py
uses its own FoundBy definition, and Pydantic requires the exact same
class instance for validation to pass.

This resolves the validation errors:
- test_cargo_fuzzer.py::test_create_finding_from_crash
- test_atheris_fuzzer.py::test_create_crash_finding

backend/tests/unit/test_modules/test_atheris_fuzzer.py

@@ -159,7 +159,7 @@ class TestAtherisFuzzerFindingGeneration:
 
     async def test_create_crash_finding(self, atheris_fuzzer):
         """Test crash finding creation"""
-        from models.finding_schema import FoundBy
+        from modules.base import FoundBy
 
         finding = atheris_fuzzer.create_finding(
             rule_id="atheris-crash",

backend/tests/unit/test_modules/test_cargo_fuzzer.py

@@ -161,7 +161,7 @@ class TestCargoFuzzerFindingGeneration:
 
     async def test_create_finding_from_crash(self, cargo_fuzzer):
         """Test finding creation"""
-        from models.finding_schema import FoundBy
+        from modules.base import FoundBy
 
         finding = cargo_fuzzer.create_finding(
             rule_id="cargo-fuzz-crash",