#2497 model validate blames foreign {{ ... }} text for an unclosed ${{ expression

What happens

An unclosed ${{ makes a later foreign {{ ... }} look like the problem. In a command/shell run such as

echo ${{ self.name } && docker ps --format '{{.Names}}'

the expression is missing a closing brace, so it runs on until the Go template's }}. Validation (#2424) then reports {{.Names}} as template text inside a dollar-double-brace expression that "cuts it short", and suggests CEL concatenation. The real fix is the missing }.

Expected

When the text between ${{ and the closing }} does not parse as CEL, say so and point at a likely unclosed expression, instead of blaming the inner {{ ... }}.

Found by the adversarial review of #2424. inside-expression in src/domain/models/template_syntax_scan.ts.