feat: Replace deprecated FastAPI on_event with lifespan event handlers

[rolodato]

Before: https://github.com/Flagsmith/edge-proxy/actions/runs/13836209807/job/38711754837

https://github.com/Flagsmith/edge-proxy/actions/runs/13820539759/job/38664556536#step:7:200

Now:

$ rye test
Running tests for edge-proxy (/Users/rolodato/source/flagsmith/edge-proxy)
================================================================ test session starts =================================================================
platform darwin -- Python 3.12.7, pytest-8.1.1, pluggy-1.4.0 -- /Users/rolodato/source/flagsmith/edge-proxy/.venv/bin/python3
cachedir: .pytest_cache
rootdir: /Users/rolodato/source/flagsmith/edge-proxy
configfile: pyproject.toml
plugins: freezegun-0.4.2, anyio-4.3.0, asyncio-0.23.6, mock-3.14.0
asyncio: mode=Mode.STRICT
collected 29 items

tests/test_environments.py::test_refresh_makes_correct_http_call PASSED                                                                        [  3%]
tests/test_environments.py::test_refresh_does_not_update_last_updated_at_if_any_request_fails PASSED                                           [  6%]
tests/test_environments.py::test_get_environment_works_correctly PASSED                                                                        [ 10%]
tests/test_environments.py::test_get_environment_raises_for_unknown_keys PASSED                                                                [ 13%]
tests/test_environments.py::test_refresh_environment_caches_clears_endpoint_caches_if_environment_changes PASSED                               [ 17%]
tests/test_environments.py::test_get_identity_flags_response_skips_cache_for_different_identity PASSED                                         [ 20%]
tests/test_environments.py::test_get_flags_response_data_skips_filter_for_server_key PASSED                                                    [ 24%]
tests/test_environments.py::test_get_flags_response_data_filters_server_side_features_for_client_key PASSED                                    [ 27%]
tests/test_environments.py::test_get_identity_flags_response_skips_filter_for_server_key PASSED                                                [ 31%]
tests/test_mappers.py::test_map_feature_state_to_response_data__return_expected PASSED                                                         [ 34%]
tests/test_mappers.py::test_map_feature_states_to_response_data__return_expected PASSED                                                        [ 37%]
tests/test_mappers.py::test_map_traits_to_response_data__return_expected PASSED                                                                [ 41%]
tests/test_models.py::test_identity_with_traits_str PASSED                                                                                     [ 44%]
tests/test_models.py::test_identity_with_traits_hash PASSED                                                                                    [ 48%]
tests/test_server.py::test_health_check_returns_200_if_cache_was_updated_recently[/proxy/health] PASSED                                        [ 51%]
tests/test_server.py::test_health_check_returns_200_if_cache_was_updated_recently[/health] PASSED                                              [ 55%]
tests/test_server.py::test_health_check_returns_500_if_cache_was_not_updated PASSED                                                            [ 58%]
tests/test_server.py::test_health_check_returns_500_if_cache_is_stale PASSED                                                                   [ 62%]
tests/test_server.py::test_health_check_returns_200_if_cache_is_stale_and_health_check_configured_correctly PASSED                             [ 65%]
tests/test_server.py::test_get_flags PASSED                                                                                                    [ 68%]
tests/test_server.py::test_get_flags_single_feature PASSED                                                                                     [ 72%]
tests/test_server.py::test_get_flags_single_feature__server_key_only_feature__return_expected PASSED                                           [ 75%]
tests/test_server.py::test_get_flags_unknown_key PASSED                                                                                        [ 79%]
tests/test_server.py::test_post_identity_with_traits PASSED                                                                                    [ 82%]
tests/test_server.py::test_post_identity__environment_with_overrides__expected_response PASSED                                                 [ 86%]
tests/test_server.py::test_post_identity__invalid_trait_data__expected_response PASSED                                                         [ 89%]
tests/test_server.py::test_get_identities PASSED                                                                                               [ 93%]
tests/test_settings.py::test_settings_are_loaded_correctly[config_file_json0-expected_config0] PASSED                                          [ 96%]
tests/test_settings.py::test_settings_are_loaded_correctly[config_file_json1-expected_config1] PASSED                                          [100%]

================================================================== warnings summary ==================================================================
.venv/lib/python3.12/site-packages/pytest_freezegun.py:17: 58 warnings
  /Users/rolodato/source/flagsmith/edge-proxy/.venv/lib/python3.12/site-packages/pytest_freezegun.py:17: DeprecationWarning: distutils Version classes are deprecated. Use packaging.version instead.
    if LooseVersion(pytest.__version__) < LooseVersion('3.6.0'):

tests/test_mappers.py::test_map_feature_state_to_response_data__return_expected
tests/test_mappers.py::test_map_feature_states_to_response_data__return_expected
tests/test_mappers.py::test_map_feature_states_to_response_data__return_expected
tests/test_models.py::test_identity_with_traits_str
tests/test_models.py::test_identity_with_traits_hash
  /Users/rolodato/source/flagsmith/edge-proxy/.venv/lib/python3.12/site-packages/pydantic/main.py:1115: PydanticDeprecatedSince20: The `parse_obj` method is deprecated; use `model_validate` instead. Deprecated in Pydantic V2.0 to be removed in V3.0. See Pydantic V2 Migration Guide at https://errors.pydantic.dev/2.7/migration/
    warnings.warn(

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
========================================================== 29 passed, 63 warnings in 2.13s ===========================================================

[matthewelwell]

@rolodato I've approved this, but I'd rather that we do one of the following (in order of preference):

1. Split out the replacement of on_event into it's own PR
2. Update the title of this PR to not be a chore. This is too big a change to be a chore.

[khvn26]

Please remove the fastapi_utils dependency.

[rolodato]

Split out the replacement of on_event into it's own PR

Done - kept the on_event change in this PR, I'll add everything else in a new one.

Update the title of this PR to not be a chore. This is too big a change to be a chore.

Edit: Changed to feat

Which would you prefer to use? I still think chore is the most representative, since we're not adding features or fixing bugs, or anything else in this list:

I agree this is a relatively risky change since we're changing how the core loop works, but I think it's still a chore at the end of the day. Happy to add functional/E2E tests if we need a different way to test this.

Please remove the fastapi_utils dependency.

Done