Assertion JiT code returning an object (rather than a bare string)
had the whole object stored into JitAssertionResult.query — a string
field — producing broken read-back and downstream encoding errors.

Coerce the result inline: string → { query }, object passes through.
Widen JitAssertionResult to `string | { query: string }` to match.

Regression tests cover the bare-object shape and an object with
additional (ignored) fields.