If you are using xslt1 then (which I assume since
rtf/result-tree-fragment don't exist in 2.0 then

<xsl:apply-templates select="document($url)" /> 

If you pass a result tree fragment to document it's like passing a
string (passing a node set would be completely different) so your
variable is coerced to a string just as if you do
<xsl:value-of select="$url"/>
if you use value-of on an empty element sucha as your request element,
you get an empty string.

You want

<xsl:variable name="query">&lt;request id="1" /&gt;</xsl:variable>

as you don't want an element node, you just want xml markup in a string
(or text node here).

David