FindSuffixThinkBlock

Here is what I found:
```json
{"name": "Rex"}

The prefix finder removes the preamble and the opening fence, so the object now
starts correctly — and parsing still fails, on the CLOSING fence, with
`Unexpected character ('`' (code 96)): expected a valid value` as the parser
looks for a second value where the input should have ended.

Measured 2026-08-10 against a local model driving a retrieval loop: it selected
the right document, scored it, and quoted a verbatim snippet — correct JSON,
discarded over a trailing backtick, three retries deep, on every request. The
failure looks exactly like an incapable model, which is why it survived so long.

Anchored on the LAST `}` so an object spanning multiple lines is kept whole.
Returns null when there is no `}` at all, rather than proposing to delete the
entire input: with no object present there is nothing to salvage, and removing
everything would turn a diagnosable parse error into an empty string.