fix(tests): Fix pre-prod eap integration tests

[wedamija]

These tests didn't inherit from the base testcase, and so were never running. I noticed this when using them as a base for similar tests for my project.

Once these tests ran in CI they ended up failing. The tests were producing TraceItems via the async Kafka producer, but SingletonProducer only flushes on process exit, so data never reached Snuba during test runs. Fix by intercepting the Kafka produce call to
capture the serialized TraceItem and inserting it via the synchronous HTTP endpoint (store_occurrences), which is the standard pattern used by all other Snuba integration tests.

Also add in missing __init__.py files.

[github-actions]

Backend Test Failures

Failures on 9d2e501 in this run:

tests/snuba/preprod/eap/test_preprod_eap_integration.py::PreprodEAPIntegrationTest::test_write_and_read_size_metric_round_trip — log

tests/snuba/preprod/eap/test_preprod_eap_integration.py:95: in test_write_and_read_size_metric_round_trip
    assert found, f"Data not found in Snuba after {max_attempts} attempts"
E   AssertionError: Data not found in Snuba after 20 attempts
E   assert False

tests/snuba/preprod/eap/test_preprod_eap_integration.py::PreprodEAPIntegrationTest::test_write_multiple_size_metrics_same_artifact — log

tests/snuba/preprod/eap/test_preprod_eap_integration.py:199: in test_write_multiple_size_metrics_same_artifact
    assert found_count == 2, f"Expected 2 records, found {found_count}"
E   AssertionError: Expected 2 records, found 0
E   assert 0 == 2

[trevor-e]

ty!