from fastapi.testclient import TestClient
import pytest
from sqlalchemy import create_engine, Column, Integer, String, DateTime, Text, func
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker

DATABASE_URL = "sqlite:///./test.db"
engine = create_engine(DATABASE_URL)
TestingSessionLocal = sessionmaker(autocommit=False, autoflush=False, bind=engine)

Base = declarative_base()

class Task(Base):
    __tablename__ = "tasks"
    id = Column(Integer, primary_key=True, index=True)
    title = Column(String(100), nullable=False)
    description = Column(Text)
    status = Column(String(20), default="pending")
    created_at = Column(DateTime, server_default=func.now())

Base.metadata.create_all(bind=engine)

def override_get_db():
    try:
        yield TestingSessionLocal()
    finally:
        pass

from main import app, get_db
app.dependency_overrides[get_db] = override_get_db

client = TestClient(app)

@pytest.fixture(autouse=True)
def clean_database():
    with engine.begin() as conn:
        conn.execute("DELETE FROM tasks")

def test_create_task():
    response = client.post("/tasks/", json={"title": "Test Task", "description": "This is a test task"})
    assert response.status_code == 201
    data = response.json()
    assert data["id"] > 0
    assert data["title"] == "Test Task"
    assert data["description"] == "This is a test task"

def test_list_tasks():
    client.post("/tasks/", json={"title": "Task 1", "description": "First task"})
    client.post("/tasks/", json={"title": "Task 2", "description": "Second task"})
    response = client.get("/tasks/")
    assert response.status_code == 200
    data = response.json()
    assert len(data) == 2

def test_get_task():
    response = client.post("/tasks/", json={"title": "Get Task", "description": "Task to get"})
    task_id = response.json()["id"]
    response = client.get(f"/tasks/{task_id}")
    assert response.status_code == 200
    data = response.json()
    assert data["id"] == task_id

def test_get_task_not_found():
    response = client.get("/tasks/999")
    assert response.status_code == 404

def test_update_task():
    response = client.post("/tasks/", json={"title": "Update Task", "description": "Task to update"})
    task_id = response.json()["id"]
    updated_data = {"title": "Updated Title"}
    response = client.put(f"/tasks/{task_id}", json=updated_data)
    assert response.status_code == 200
    data = response.json()
    assert data["id"] == task_id
    assert data["title"] == "Updated Title"

def test_update_task_not_found():
    updated_data = {"title": "Updated Title"}
    response = client.put("/tasks/999", json=updated_data)
    assert response.status_code == 404

def test_delete_task():
    response = client.post("/tasks/", json={"title": "Delete Task", "description": "Task to delete"})
    task_id = response.json()["id"]
    response = client.delete(f"/tasks/{task_id}")
    assert response.status_code == 204
    response = client.get(f"/tasks/{task_id}")
    assert response.status_code == 404

def test_delete_task_not_found():
    response = client.delete("/tasks/999")
    assert response.status_code == 404

This `test_main.py` file includes tests for all CRUD operations using the FastAPI TestClient. Each test creates its own data and ensures that there are no dependencies between tests. The database is reset before each test to ensure isolation.