chore: update development dependencies and improve test coverage

- Upgraded `pytest-asyncio` to version 1.0 to ensure compatibility with the latest features and improvements.
- Increased the coverage threshold in pytest configuration to 80%, enhancing the quality assurance process.
- Added a new `conftest.py` file to manage shared fixtures and improve test organization.
- Introduced multiple new test files to cover various components, ensuring comprehensive test coverage across the application.
- Updated the `.coverage` file to reflect the latest coverage metrics.

pyproject.toml

@@ -25,7 +25,7 @@ duty-teller = "duty_teller.run:main"
 [project.optional-dependencies]
 dev = [
     "pytest>=8.0,<9.0",
-    "pytest-asyncio>=0.24,<1.0",
+    "pytest-asyncio>=1.0,<2.0",
     "pytest-cov>=6.0,<7.0",
     "httpx>=0.27,<1.0",
 ]
@@ -49,8 +49,11 @@ line-length = 120
 target-version = ["py311"]
 
 [tool.pytest.ini_options]
-addopts = "--cov=duty_teller --cov-report=term-missing --cov-fail-under=51"
+addopts = "--cov=duty_teller --cov-report=term-missing --cov-fail-under=80"
 asyncio_mode = "auto"
+filterwarnings = [
+    "ignore::DeprecationWarning:pytest_asyncio.plugin",
+]
 
 [tool.pylint.messages_control]
 disable = ["C0114", "C0115", "C0116"]