Add test support for CentOS Stream 9

Adds test resources and test case for CentOS Stream 9, the upstream
development platform for RHEL 9. Includes centos-release and os-release
files for proper distribution identification.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: nir0s

tests/resources/distros/centosstream9/etc/centos-release

@@ -0,0 +1 @@
+CentOS Stream release 9

tests/resources/distros/centosstream9/etc/os-release

@@ -0,0 +1,14 @@
+NAME="CentOS Stream"
+VERSION="9"
+ID="centos"
+ID_LIKE="rhel fedora"
+VERSION_ID="9"
+PLATFORM_ID="platform:el9"
+PRETTY_NAME="CentOS Stream 9"
+ANSI_COLOR="0;31"
+LOGO="fedora-logo-icon"
+CPE_NAME="cpe:/o:centos:centos:9"
+HOME_URL="https://centos.org/"
+BUG_REPORT_URL="https://issues.redhat.com/"
+REDHAT_SUPPORT_PRODUCT="Red Hat Enterprise Linux 9"
+REDHAT_SUPPORT_PRODUCT_VERSION="CentOS Stream"

tests/test_distro.py

@@ -464,6 +464,19 @@ def test_rocky9_os_release(self) -> None:
         }
         self._test_outcome(desired_outcome)
 
+    def test_centosstream9_os_release(self) -> None:
+        desired_outcome = {
+            "id": "centos",
+            "name": "CentOS Stream",
+            "pretty_name": "CentOS Stream 9",
+            "version": "9",
+            "pretty_version": "9",
+            "best_version": "9",
+            "like": "rhel fedora",
+            "codename": "",
+        }
+        self._test_outcome(desired_outcome)
+
     def test_slackware14_os_release(self) -> None:
         desired_outcome = {
             "id": "slackware",