Add DD_PRIVATE_ACTION_RUNNER_SPLIT_ENABLED env var

[julesmcrt]

Summary

• Apply Gabriel Plassard’s change to support DD_PRIVATE_ACTION_RUNNER_SPLIT_ENABLED during initial Agent installation.
• Write private_action_runner.split_enabled when explicitly provided (true or false); omit it otherwise to preserve the Agent’s platform default.
• Add tests for enabled, disabled, and unspecified split mode. Existing configurations remain untouched.

Validation

• All 64 shell unit tests passed in a Debian Docker container with the Agent installed.
• ShellCheck passed over the CI script set (excluding the unit-test-generated extracted_functions.sh artifact).
• make, Bash syntax checks, and git diff --check passed.

Co-authored with @gplassard.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-02T11:56:06.907585Z	92a65a5	PR opened

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[datadog-prod-us1-5]

✅ All CI checks and tests passed.

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

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