[bug] when using a validator with a default for nested data it parses value via toml twice

[rochacbruno]

Describe the bug

from __future__ import annotations

from dynaconf import Dynaconf
from dynaconf import Validator

settings = Dynaconf()
settings.validators.register(
    Validator("group.something_new", default=5),
)
settings.validators.validate()

assert settings.group.test_list == ["1", "2"], settings.group

Execution

$ DYNACONF_GROUP__TEST_LIST="['1','2']" python app.py

expectation:

settings.group.test_list == ["1", "2"]

current behavior

$ DYNACONF_GROUP__TEST_LIST="['1','2']" python app.py
Traceback (most recent call last):
  File "/home/rochacbruno/Projects/dynaconf/tests_functional/issues/905_item_duplication_in_list/app.py", line 21, in <module>
    assert settings.group.test_list == ["1", "2"], settings.group
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: {'something_new': 5, 'TEST_LIST': [1, 2]}

What is happening?

At the end of validators.validate.validate the Validator("group.something_new", default=5), is executed and then default value is merged with current data, during that process dynaconf is calling parse_conf_data passing tomlfy=True which forces toml to evaluate the existing data again so.

In [1]: from dynaconf.utils.parse_conf import parse_conf_data

In [2]: data = {"TEST_LIST": ["1", "2"]}

In [3]: parse_conf_data(data, tomlfy=True)
Out[3]: {'TEST_LIST': [1, 2]}

In [4]: parse_conf_data(data, tomlfy=False)
Out[4]: {'TEST_LIST': ['1', '2']}

Possible solutions:

Change setdefault method to use tomlfy=False
Change parse_with_toml to avoid doing that transformation.