fix(config): accept the wider additional_endpoints input forms

[webern]

Human Summary

Caught by AI when reviewing #1993, it looks like this was a regression in the environment variable path only, which should not have affected customers anyway, but it looks like now we handle JSON in environment variable correctly if we happen to ever read that.

AI Summary

Restores the wider additional_endpoints input shapes that were lost when the metrics encoder and
forwarder moved off the component serde onto the typed model.

The old AdditionalEndpoints serde used PickFirst<(DisplayFromStr, _)> plus OneOrMany, so it
accepted:

• the whole map as a JSON string — the form an environment variable produces, e.g.
  DD_ADDITIONAL_ENDPOINTS='{"https://app.datadoghq.com":["key"]}', and
• a bare string in place of a one-element key list for a host.

The generated DatadogConfiguration deserializes additional_endpoints as a plain
HashMap<String, Vec<String>>, and the env overlay only materializes scalar/space-separated-list
env values, so neither shape survives. Both now fail deserialization outright — a dual-shipping
deployment configured through the environment fails to start.

This adds an additional_endpoints case to normalize_datadog_input_forms (the same pass that
already restores the dogstatsd_eol_required and dogstatsd_mapper_profiles env-var forms): parse
the JSON-string form into an object, then wrap any bare-string host value into a one-element array.
Invalid JSON is left in place so the downstream deserializer still surfaces the error.

Change Type

• Bug fix

How did you test this PR?

• cargo nextest run -p agent-data-plane-config-system (added additional_endpoints_accepts_json_string_scalar_or_map, covering the JSON-string, JSON-string-with-scalar, native-scalar, and native-list shapes; confirmed it fails without the fix with invalid type: string ..., expected a map).
• make fmt

References

• Merges into m/pr5-cutover

[Copilot]

Pull request overview

Fixes a regression in config deserialization by re-introducing legacy additional_endpoints input shapes during the Datadog config normalization step, ensuring environment-variable–sourced configurations continue to load correctly in dual-shipping deployments.

Changes:

• Extend normalize_datadog_input_forms to accept additional_endpoints as either a JSON string (env-var form) or a map whose values may be scalar strings (wrapped into a 1-element list).
• Add a focused async test covering JSON-string map, JSON-string-with-scalar, native map-with-scalar, and native map-of-lists shapes.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.