test: assert fmt is idempotent in the integration queries

[prql-bot]

The fmt integration test snapshotted the formatted query and then only checked that the output still parsed (prqlc::prql_to_pl(&formatted).unwrap()) — a formatter that reshaped an already-formatted query on a second pass would pass that check. This replaces it with a full round-trip: re-parse and re-format the output and assert it reproduces itself exactly, which subsumes the parse check.

All 31 query files already satisfy the stronger property, so this is a regression guard rather than a bug fix — no formatter behavior changes and no snapshots move. It costs one extra parse/format per query file (the whole integration target still runs in ~11s locally).

Verification

$ cargo test -p prqlc --test integration fmt::
test result: ok. 31 passed; 0 failed; 0 ignored; 0 measured; 463 filtered out
$ cargo test -p prqlc --test integration
test result: ok. 489 passed; 0 failed; 5 ignored; 0 measured; 0 filtered out

similar_asserts::assert_eq! is used rather than assert_eq! so a future failure prints a diff of the two formattings instead of two full query dumps; it is already a dev-dependency of this target, used by the results module.

Context: idempotency is the property most at risk in the open formatter work — #2757, #4116, and #6085 all sit in fmt.