test(auto-paginate): fix over-strict cursor contract at exact page boundary

[BYK]

Summary

Fixes a flaky property test, auto-paginate.property.test.ts →
"drops nextCursor whenever the result is trimmed to limit", which failed on
seeds that hit an exact page-boundary input (e.g. counterexample
[total=393, limit=392, pageSize=98], since 392 % 98 === 0).

This is a test-only fix — the autoPaginate() implementation is correct.

Root cause

The test asserted that whenever total > limit, nextCursor must be
undefined. That predicate conflates two cases with opposite, correct
contracts in the multi-page path:

• True overshoot — a page pushes accumulation strictly past limit. Rows are
  trimmed and the server cursor points past the trimmed tail, so it is dropped.
  Preserving it would skip rows (the original skip-bug guard). ✅
• Exact boundary (limit % effectivePageSize === 0) — accumulation lands
  exactly on limit, nothing is trimmed, and the last page's cursor points at
  index limit (the first unreturned row). It must be preserved so callers
  can page forward; dropping it strands the tail — the regression explicitly
  pinned by events-overshoot.test.ts. ✅

In the counterexample, autoPaginate correctly returns nextCursor="392"
(following it yields row 392 — contiguous, no skip), but the test demanded
undefined.

Fix

Split the assertion by the page-boundary discriminator:

• limit % effectivePageSize === 0 (exact boundary) → assert
  nextCursor === String(limit) (positively verifies the cursor resumes
  contiguously).
• otherwise (true overshoot) → assert nextCursor === undefined (skip-bug
  guard, unchanged).

Also updated the file header to document the boundary nuance.

Why not change the implementation?

Dropping the cursor at the exact boundary would reintroduce the stranding
regression that events-overshoot.test.ts guards against (-c next could never
advance past limit). The implementation's two-branch logic is correct.

Verification

• Exhaustive sweep of all (total, limit, pageSize) combinations in the
  test's arbitrary ranges (103,171 cases): 0 violations against the new
  contract.
• auto-paginate.property.test.ts + events-overshoot.test.ts: pass.
• Property test run 15× consecutively: 15/15 green (was intermittently
  failing).
• pnpm run lint: clean. pnpm exec tsc --noEmit: exit 0.

Context

This flake was spotted during the full-suite run while fixing the response-cache
torn-read bug (#1056) and noted there as a follow-up.

[github-actions]

Codecov Results 📊

✅ Patch coverage is 100.00%. Project has 4420 uncovered lines.
✅ Project coverage is 81.85%. Comparing base (base) to head (head).

Coverage diff

@@            Coverage Diff             @@
##          main       #PR       +/-##
==========================================
+ Coverage    81.85%    81.85%        —%
==========================================
  Files          341       341         —
  Lines        24358     24358         —
  Branches     15912     15912         —
==========================================
+ Hits         19937     19938        +1
- Misses        4421      4420        -1
- Partials      1665      1664        -1

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