refactor: consolidate redundant spireEnabled checks

[Alan-Cha]

Summary

Consolidates two consecutive if spireEnabled blocks in BuildEnvoyProxyContainerWithSpireOption into a single conditional block.

Changes

• Merged lines 111-117 and 119-128 in container_builder.go
• Both volume mounts (svid-output and spiffe-helper-config) now added in a single append call
• Preserved the comment explaining why spiffe-helper-config is needed

Testing

• ✅ All existing tests pass
• No functional changes, purely refactoring for readability

Before

if spireEnabled {
    volumeMounts = append(volumeMounts, corev1.VolumeMount{...})
}

if spireEnabled {
    volumeMounts = append(volumeMounts, corev1.VolumeMount{...})
}

After

if spireEnabled {
    volumeMounts = append(volumeMounts,
        corev1.VolumeMount{...},
        corev1.VolumeMount{...},
    )
}

Assisted-By: Claude Code