fix(telemetry): drop the stale >= 0 floor on the signed saved-token sums

`tokens_saved_24h` and `total_tokens_saved` both return `i64` from an unclamped
`SUM(saved_tokens)` -- deliberately, unlike the `as usize` readers that take
`.max(0)` for their unsigned API. A window whose filters emitted more than they
saved sums honestly negative, so `assert!(saved_24h >= 0)` panics on any DB
holding a net-negative row.

No ordering invariant replaces the floor: with a positive row inside the 24h
window and a larger negative one outside it, saved_24h (90) exceeds saved_total
(-110). Nothing about either value is assertable, so the test stops binding them.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>