Add to Templating Engine Docs: InputParameter inline-scripts using templating engines fail to resolve when starting with ${ or #{

[user170200]

I believe this behavior is rather curious, than an actual bug - Which is why I think the documentation should mention it

A simple example of an activity with input mappings below:

• working = all is fine
• failing = fails, because UEL does not do freemarker magic and fails to resolve '${ict}'
• workaround = prepend with a comment so 'isExpression' evaluates to false

<bpmn:sendTask id="Act1" name="doesn't matter">
  <bpmn:extensionElements>
    <camunda:inputOutput>
      <camunda:inputParameter name="working">
        <camunda:script scriptFormat="freemarker">Hello ${name}, I heard you like:
&lt;#list icecreamtypes as ict&gt;
  * ${ict}
&lt;/#list&gt;
      </camunda:inputParameter>
      <camunda:inputParameter name="failing">
        <camunda:script scriptFormat="freemarker">${name} likes:
&lt;#list icecreamtypes as ict&gt;
  * ${ict}
&lt;/#list&gt;
      </camunda:inputParameter>
      <camunda:inputParameter name="workaround">
        <camunda:script scriptFormat="freemarker">&lt;#-- leading comment prevents Operaton from treating this as a dynamic script-source expression (see StringUtil.isExpression) --&gt;${name} likes:
&lt;#list icecreamtypes as ict&gt;
  * ${ict}
&lt;/#list&gt;
      </camunda:inputParameter>
    </camunda:inputOutput>
  </bpmn:extensionElements>
</bpmn:sendTask>

The reason is, that the input value is checked to be a dynamicExpression:
https://github.com/operaton/operaton/blob/main/engine/src/main/java/org/operaton/bpm/engine/impl/util/ScriptUtil.java#L195,
which calls:
https://github.com/operaton/operaton/blob/b272acfa5c6486e4a0e0b2ef89a692a64b65c423/commons/utils/src/main/java/org/operaton/commons/utils/StringUtil.java#L36

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Suggestion:

• Add to Templating Engine Docs, that starting a template expression with ${ or #{ leads to misinterpretation, which can be fixed by adding a comment or starting with a different character sequence.