Drive real traffic through the proxy and read the events back out of the
Datadog Logs Search API to prove delivery instead of trusting the proxy. The
DatadogClient reads DD_API_KEY, DD_SITE and DD_APP_KEY from the environment and
polls the search API for a per-run marker stamped into each request
Three scenarios cover the contract: a batch of gpt-5.5 chat completions shipped
with no drops, the /v1/responses path on a claude-haiku-4-5 deployment
exercising the Responses-to-Anthropic translation, and a failed request that
must land as an error event. The e2e compose config enables the datadog (and
prometheus) callback so the delivery path is actually exercised, and the suite
skips unless all three credentials are set
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