Recognize ${braced} variables in template extract_vars

[c-tonneslan]

Template.extract_vars only collects the named group from string.Template's match:

return [
    match.group("named")
    for match in string_template.pattern.finditer(string_template.template)
    if match.group("named")
]

But string.Template matches a variable as either $named or ${braced}, and both forms are valid. So ${var} placeholders are invisible to extract_vars, with two consequences:

• Template.vars() under-reports — Template(name="x", prompt="Hello ${name}").vars() returns set() instead of {"name"}.
• interpolate()'s missing-variable check never sees them, so a template using ${missing} raises a raw KeyError out of string.Template.substitute() instead of the intended Template.MissingVariables error.

>>> Template.interpolate("Hi ${who}", {})
KeyError: 'who'          # expected: MissingVariables: Missing variables: who

Fix collects the braced group too. Added ${braced} cases (resolve, missing, and mixed $a + ${b}) to the existing test_template_evaluate parametrize list — the missing-variable case fails on main with a raw KeyError and passes with the fix. Full test_templates.py (49 tests) green.