Fix add_field_serializer_for_reverse_relations clearing validators (#1302)

* Fix add_field_serializer_for_reverse_relations clearing validators

* add test to check that validators are not removed

* compatibility with old python

* fix test default values

* fix coverage and cleanup

---------

Co-authored-by: collerek <collerek@gmail.com>

tests/test_relations/test_reverse_relation_preserves_validator.py

@@ -0,0 +1,48 @@
+from typing import List, Optional, Union
+
+import ormar
+import pytest_asyncio
+from pydantic import field_validator
+
+from tests.lifespan import init_tests
+from tests.settings import create_config
+
+base_ormar_config = create_config()
+
+
+class Author(ormar.Model):
+    ormar_config = base_ormar_config.copy()
+
+    id: int = ormar.Integer(primary_key=True)
+    name: str = ormar.String(max_length=80)
+
+    @field_validator("name", mode="before")
+    @classmethod
+    def validate_name(cls, v: Union[str, List[str]]) -> str:
+        if isinstance(v, list):
+            v = " ".join(v)
+        return v
+
+
+class Post(ormar.Model):
+    ormar_config = base_ormar_config.copy()
+
+    id: int = ormar.Integer(primary_key=True)
+    title: str = ormar.String(max_length=200)
+    author: Optional[Author] = ormar.ForeignKey(Author)
+
+
+create_test_database = init_tests(base_ormar_config)
+
+
+@pytest_asyncio.fixture(scope="function", autouse=True)
+async def cleanup():
+    yield
+    async with base_ormar_config.database:
+        await Post.objects.delete(each=True)
+        await Author.objects.delete(each=True)
+
+
+def test_validator():
+    author = Author(name=["Test", "Author"])
+    assert author.name == "Test Author"