JSON template preview doesn't unpack values

[LucasPickering]

Unpacked JSON values are not templated correctly. Take this body as an example:

body:
  type: json
  data:
    {
      "age": "{{ prompt(message='Age', default=3) | integer() }}",
      "weight_kg": "{{ prompt(message='Weight (kg)', default='1.5') | float() }}",
    }

The age field should be unpacked to an integer. That happens correctly in request that's actually sent:

{"age":3,"weight_kg":1.5}

But here's what you see in the preview pane before sending:

{
  "age": "3",
  "weight_kg": "1.5"
}

[LucasPickering]

If we replace implicit unpacking with explicit !expr clauses, this will be easier to read/reason about in YAML and might be easier to handle internally.

body:
  type: json
  data:
    {
      "age": !expr "prompt(message='Age', default=3) | integer()",
      "weight_kg": !expr "prompt(message='Weight (kg)', default='1.5') | float()",
    }

I initially avoided this because I wanted to avoid YAML tags. The YAML language server doesn't have great support for tags, so this doesn't play nicely with the JSONSchema support. It may be worth the tradeoff though. By explicitly declaring this is an expression, we:

• Avoid the seeming randomness of unpacking, where whitespace can easily break the unpack
• Make it clear that this is an expression that will be unpacked
• Remove extra curly braces, which provide no semantic value
• Make it clear internally when an expression is going to be unpacked, rather than having to intuit in TemplatePreview based on the downstream usage.
• Makes it possible to unpack expressions in other contexts, such as profiles