Some `gcp` tests fail

[pront]

The failing tests are currently ignored.

./scripts/run-integration-test.sh  int gcp

[pront]

As of #25670, the two ignored tests (publish_events, publish_invalid_events) have been removed and the timberio/chronicle-emulator Docker image dependency has been dropped. The invalid_credentials test was converted to a self-contained unit test using wiremock.

What still needs to be done: add tests that verify end-to-end event delivery and 4xx rejection handling for the gcp_chronicle_unstructured sink. Viable approaches:

1. Extend the existing wiremock-based tests — mock the Chronicle Ingestion API endpoint to return 200/400 and assert sink compliance/error. No external service needed; coverage is at the HTTP boundary.
2. Write a minimal Chronicle API stub — a small HTTP server that implements the unstructured log import endpoint, stores payloads in memory, and exposes a retrieval endpoint. Can be packaged as a Docker image and slotted back into compose.yaml.
3. Real Chronicle staging environment — point the tests at a GCP project with Chronicle enabled using CI-managed service account credentials. Most faithful but requires GCP setup.

Option 1 is the lowest-friction starting point and likely sufficient for catching regressions at the serialization/auth layer.