Handle Red Hat-family hosts without /etc/redhat-release

[swang392]

What does this PR do?

Fixes Red Hat-family release detection when /etc/redhat-release is missing.

Some images can still be classified as Red Hat-family through /etc/system-release, /etc/os-release, or /usr/lib/os-release, but the install path later parsed the major version only from /etc/redhat-release. When that file is absent, bash prints No such file or directory before the script falls back to 7.

This PR adds a helper that checks release metadata in order:

• /etc/redhat-release
• /etc/system-release
• /etc/os-release
• /usr/lib/os-release
• existing fallback to 7

This keeps the existing behavior for normal RHEL/CentOS hosts and preserves the old fallback for hosts where none of the metadata files provide a numeric version.

Why?

TON-881 tracks an MCI install failure where the public Agent 7 install script reports missing /etc/redhat-release on a Red Hat-family host. The fix belongs here because MCI downloads the generated install_script_agent7.sh at runtime.

Testing

• python3 unit_tests/extract_functions.py
• focused no-sudo harness for get_redhat_release_version covering /etc/redhat-release, /etc/system-release, /etc/os-release, /usr/lib/os-release, nonnumeric VERSION_ID, fallback to 7, and missing-file stderr silence
• make

I also tried ./unit_tests/test_install_script.sh locally with a temporary shunit2, but the full suite exercises existing sudo-based tests against /etc/datadog-agent/...; this machine is not configured for passwordless sudo, so I stopped it after sudo failures unrelated to this change.

Notes

This fixes the confirmed missing release metadata handling. It does not change yum repo URL construction or publish regenerated scripts; release/S3 publishing should happen through the normal follow-up flow.

[datadog-datadog-prod-us1-2]

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

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