Fix ID/UUID field serialization and Pydantic type mapping (#40)

[AghastyGD]

Issue: #40

Problem
Django models using UUIDField, UUIDAutoField, or ForeignKey with a UUID PK were not properly serialized in lazy-ninja.
Pydantic schemas expected int or str, causing validation errors when UUIDs were returned directly.

Solution

• Update get_pydantic_type to check UUIDField before AutoField, ensuring UUIDAutoField maps to str.
• Update serialize_model_instance to convert UUIDField and ForeignKey (to UUID PKs) into strings.
• Enhance convert_foreign_keys to handle both integer and string PKs.
• Add parse_model_int, a new helper that converts string path parameters into the correct type for the model’s PK in CRUD routes.

Impact

• Fixes Pydantic validation errors when returning UUID primary keys from models.
• Supports projects that have DEFAULT_AUTO_FIELD = "core.fields.UUIDAutoField".
• No breaking changes for models using standard AutoField PKs.

Example

class User(models.Model):
    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
    name = models.CharField(max_length=100)

class Post(models.Model):
    id = models.AutoField(primary_key=True)
    author = models.ForeignKey(User, on_delete=models.CASCADE)

# Post instance serialization
serialize_model_instance(post_instance)
# Returns: {'id': 1, 'author': 'd290f1ee-6c54-4b01-90e6-d701748f0851'}

Closes #40