Support Template in XML Body

Contrary to JSON body, the template body should still a valid XML.
Variables are encapsulated within an XML atttribute and XML text content.

<user name="{{name}}"/>
<value>{{value}}</value>

Similarly to JSON, value should be escaped automatically.

[Variables]
delimiter="\u{0022}"   # this is the double-quote
count=10
status=true

# JSON sent
# {
#   "delimiter": "\"",
#   "count": 10,
#   "status": true
# }
POST http::/localhost:8000/json
{
  "delimiter": "{{delimiter}}",
  "count": {{count}},
  "status": {{status}}
}

# XML sent
# <variables>
# <variable name="delimiter" value="&#34;"/>
#  <variable name="count" value="10"/>
#  <variable name="status" value="true"/>
# </variables>
POST http::/localhost:8000/xml
<variables>
  <variable name="delimiter" value="{{delimiter}}"/>
  <variable name="count" value="{{count}}"/>
  <variable name="status" value="{{status}}"/>
</variables>








It should be consistent with the existing template support in JSON body.

The issue names no implementation files or tests; start by locating the existing JSON body template handling in the request-body path. Trace how variables are substituted, then check the XML examples for attribute and text-content escaping. Done means XML templates remain valid after substitution and behave consistently with JSON templates.