Dear Vitaliy and contributors! Thank you very much for a wonderful project!

Describe the bug
When creating a ModelSchema with declared fields, the behavior differs depending on whether fields or exclude is specified in the Meta class:

• With fields: Declared fields are correctly preserved and model fields are filtered
• With exclude: Declared fields are incorrectly removed from the schema

Versions (please complete the following information):

• Python version: 3.14.0
• Django version: 6.0
• Django-Ninja version: 1.5.1
• Pydantic version: 2.12.5

Note you can quickly get this by runninng in ./manage.py shell this line:

from django.db import models

from ninja.orm import ModelSchema
from pydantic import Field


class StatusChoice(models.IntegerChoices):
    NEGATIVE = 0, "Negative"
    POSITIVE = 1, "Positive"
    NOT_ASSESSED = 98, "Not assessed"
    UNK = 99, "Unknown"


class Response(models.Model):
    status = models.IntegerField(
        choices=StatusChoice, default=StatusChoice.NOT_ASSESSED
    )

class ResponseSchemaField(ModelSchema):
    status: str = Field(alias="get_status_display")
    class Meta:
        model = Response
        fields = ["id"]

class ResponseSchemaExclude(ModelSchema):
    status: str = Field(alias="get_status_display")
    class Meta:
        model = Response
        exclude = ["id"]

instance = Response.objects.create()
ResponseSchemaField.from_orm(instance)  # ✓ ResponseSchemaField(status='Not assessed', id=4)
ResponseSchemaExclude.from_orm(instance)  # ✗ ResponseSchemaExclude(status=98)

Expected Behavior
Both fields and exclude should respect explicitly declared fields in the schema class and only filter auto-generated model fields.

Actual Behavior
Using exclude removes declared fields from the generated schema, causing them to be replaced by model fields of the same name.

However, _selected_model_fields returns all fields when using exclude, causing declared fields to be overwritten.