fix: improve userns validation when joining pods

- remove old CLI validation that only checked --pod flag
- add validation in namespaces.go to catch all paths (cli, quadlet, api)
- block userns mixing for all pods with infra, not just ipc/net
- update error message to be clearer
- fix test cleanup to use PodmanExitCleanly()
- use dynamic pod names in system tests to avoid conflicts

fixes #26848

Signed-off-by: 0xdvc <neilohene@gmail.com>

test/e2e/pod_create_test.go

@@ -803,7 +803,7 @@ ENTRYPOINT ["sleep","99999"]
 		// fail if --pod and --userns set together
 		session = podmanTest.Podman([]string{"run", "--pod", podName, "--userns", "keep-id", ALPINE, "id", "-u"})
 		session.WaitWithDefaultTimeout()
-		Expect(session).Should(ExitWithError(125, "--userns and --pod cannot be set together"))
+		Expect(session).Should(ExitWithError(125, "cannot set user namespace mode when joining pod with infra container"))
 	})
 
 	It("podman pod create with --userns=keep-id can add users", func() {