Validation on poorly formed template variable

[squeedee]

Description of problem

I can use template interpolation with errors in it, and I don't see a validation error when I feel i could, eg:

myThing: $(my.var) # missing closing $
myThing: $(my.var # missing closing )$

Proposed solution

Given I omit the template closing `$` or `)$` on a line
When I submit the template
Then the API server rejects my template

Additional Context

Requires some syntax assertions about single line template vars (no line splitting)