Fix DDOT install for Agent >= 7.78.0

[BaptisteFoy]

Bug

For Agent >= 7.78.0, DDOT is installed by the Agent's own post-install script rather than the install script. The previous fix (#406) handled this by unsetting `DD_OTELCOLLECTOR_ENABLED`, which caused two regressions:

1. `DD_OTELCOLLECTOR_ENABLED` was no longer present in the environment when running `apt-get`/`yum`/`zypper install datadog-agent`, so the Agent's post-install script couldn't see it and wouldn't install DDOT
2. `update_ddot` and `manage_otel_config` were also skipped (they gate on `DD_OTELCOLLECTOR_ENABLED`), so the Datadog Agent config was not updated for DDOT

Why not pass DD_OTELCOLLECTOR_ENABLED to the package manager?

An intermediate approach passed `DD_OTELCOLLECTOR_ENABLED` to the `apt-get`/`yum`/`zypper` install command so the Agent's post-install script could pick it up. This failed because the Agent post-install runs before the install script creates `datadog.yaml`, causing:

failed to install extensions: extension ddot: could not install extension: failed to run hook (postInstallExtension): run failed: failed to write otel-config.yaml: failed to read datadog.yaml: open /etc/datadog-agent/datadog.yaml: no such file or directory

Fix

For Agent >= 7.78.0, instead of relying on the Agent post-install script:

• Do not pass `DD_OTELCOLLECTOR_ENABLED` to the package manager (avoids triggering the broken post-install)
• After `datadog.yaml` is created by the install script, call `datadog-installer extension install "oci://${ddot_oci_registry}/agent-package:${ddot_oci_tag}" "ddot"` directly — this handles both DDOT installation and configuration
• Skip the `datadog-agent-ddot` deb/rpm package install and repo setup (no longer needed)

For Agent < 7.78.0, behavior is unchanged: the `datadog-agent-ddot` package is installed from the dedicated apt/yum/zypper repo, and `update_ddot`/`manage_otel_config` run as before.

Test plan

• With `DD_OTELCOLLECTOR_ENABLED` set and Agent < 7.78.0: ddot repo setup and package install run as before, `update_ddot`/`manage_otel_config` still run
• With `DD_OTELCOLLECTOR_ENABLED` set and Agent >= 7.78.0: ddot repo/package install are skipped, `datadog-installer extension install` is called after `datadog.yaml` is created
• Without `DD_OTELCOLLECTOR_ENABLED`: no behavior change

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 0c3987a7e7

ℹ️ About Codex in GitHub

Codex has been enabled to automatically review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

When you sign up for Codex through ChatGPT, Codex can also answer questions or update the PR, like "@codex address that feedback".

[liustanley]

Do we also need to set DD_INSTALLER_REGISTRY_URL_AGENT_PACKAGE on the install commands? We're using DD_INSTALLER_REGISTRY_URL_AGENT_PACKAGE for ddot_oci_registry but this env var isn't passed to the install commands. So if DD_INSTALLER_REGISTRY_URL_AGENT_PACKAGE is set without DD_INSTALLER_REGISTRY_URL, we get the following error in upgrades/downgrades:

could not download image using install.datadoghq.com/agent-package:7.78.0-rc.2-1: GET https://install.datadoghq.com/v2/agent-package/manifests/7.78.0-rc.2-1: unexpected status code 404 Not Found: <?xml version="1.0" encoding="UTF-8"?>
<Error><Code>NoSuchKey</Code><Message>The specified key does not exist.</Message><Key>v2/agent-package/manifests/7.78.0-rc.2-1</Key><RequestId>EKNAE187CXD0RGH9</RequestId><HostId>zKo74kJNoNUgVjJfMUt7hjw+WGUoiyZ4lbB2E1x04XKbAhoiLxEKmqDos7a+qyZYXffbCInF2bI=</HostId></Error>
could not download image using install.datadoghq.com/agent-package:7.78.0-rc.2-1: GET https://gcr.io/v2/datadoghq/agent-package/manifests/7.78.0-rc.2-1: MANIFEST_UNKNOWN: Failed to fetch "7.78.0-rc.2-1"
failed to restore extensions: could not install extensions: could not download package: could not download package: could not download image from any registry: could not download image using install.datadoghq.com/agent-package:7.78.0-rc.2-1: GET https://install.datadoghq.com/v2/agent-package/manifests/7.78.0-rc.2-1: unexpected status code 404 Not Found: <?xml version="1.0" encoding="UTF-8"?>
<Error><Code>NoSuchKey</Code><Message>The specified key does not exist.</Message><Key>v2/agent-package/manifests/7.78.0-rc.2-1</Key><RequestId>EKNAE187CXD0RGH9</RequestId><HostId>zKo74kJNoNUgVjJfMUt7hjw+WGUoiyZ4lbB2E1x04XKbAhoiLxEKmqDos7a+qyZYXffbCInF2bI=</HostId></Error>; could not download image using install.datadoghq.com/agent-package:7.78.0-rc.2-1: GET https://gcr.io/v2/datadoghq/agent-package/manifests/7.78.0-rc.2-1: MANIFEST_UNKNOWN: Failed to fetch "7.78.0-rc.2-1"
W: --force-yes is deprecated, use one of the options starting with --allow instead.

[liustanley]

Did some testing and found that the rpm -> oci upgrade was failing

Suggested change

	# When upgrading to 7.78+, DDOT is managed via OCI extension install.
	# Remove the legacy datadog-agent-ddot RPM first to avoid strict version
	# dependency conflicts that prevent yum from upgrading datadog-agent.
	if [ -n "$ddot_use_installer" ] && rpm -q datadog-agent-ddot &>/dev/null; then
	echo -e " \033[33mRemoving legacy datadog-agent-ddot RPM package before upgrade...\n\033[0m"
	$sudo_cmd yum -y remove datadog-agent-ddot
	fi