test: close the adapter e2e coverage gap (datadog, hyperdx, otlp, fs)

[HugoRCD]

Current state

pnpm run test:e2e covers 6 of the 11 shipped adapters:

Adapter	e2e	Note
axiom	✅	round-trip when the token has query:read
posthog	✅	smoke (write-only API)
sentry	✅	smoke (DSN is write-only)
better-stack	✅	smoke (source token is write-only)
loki	✅	round-trip, local container or Grafana Cloud
clickhouse	✅	round-trip, local container or ClickHouse Cloud
datadog	❌
hyperdx	❌
otlp	❌
fs	❌

These suites are the safety net for "the destination quietly changed its API". Four adapters currently have none.

Suggested order

1. fs — do this one first. Fully local, no secrets, no container. It can be a true round-trip: write events, read the NDJSON back, assert rotation / maxFiles / maxSizePerFile behaviour. Unlike every other suite it can run unconditionally in CI on every PR, not just the nightly cron.

2. otlp — extend the sandbox. Add an otel/opentelemetry-collector service to packages/evlog/test/e2e/docker-compose.yml with a file exporter, then assert the exported records. No account needed, and it validates the OTLP payload shape that hyperdx and posthog (logs mode) both reuse.

3. hyperdx — smoke against the cloud endpoint, or point it at the same local collector since createHyperDXDrain() is a thin mapping onto sendBatchToOTLP().

4. datadog — smoke; the intake API is write-only, so this mirrors the Better Stack suite. Worth asserting the httpStatusCode renaming and the dd.trace_id block survive the wire.

Pattern to follow

test/e2e/loki.e2e.ts and test/e2e/clickhouse.e2e.ts are the round-trip references; test/e2e/better-stack.e2e.ts is the smoke reference. All use describeIfEnv() so a missing endpoint skips with a visible label instead of passing silently, and makeEvent() to tag every event with e2e_run_id / e2e_correlation_id for cleanup.

Adapters with a local container should be wired into sandbox:up / sandbox:e2e so they run without credentials.

Also

.github/workflows/e2e.yml and test/e2e/README.md list the required secrets — both need the new entries.