fix(#496): reject oversized variable option probes

[BorisTyshkevich]

What & why

Closes #496.

Reject a successful Dashboard-variable option probe when its raw response
contains the 1001st sentinel row. Column shape is validated first, while server
errors and cancellation remain untouched; overflow is assigned before History
or detached-result source bookkeeping can treat the run as successful.

The verdict is based only on the returned row count, so zero through 1000 rows
pass with or without authored LIMIT syntax. The existing probe bound,
Dashboard batch bounds, byte cap, read-only transport, and truncation behavior
are unchanged.

Verification

• npm test — 193 files / 6,433 tests; 100% statements and lines
• npm run build
• npm run test:e2e -- --project=chromium --project=webkit tests/e2e/variable-tab.spec.js — 24 passed
• Full E2E additionally surfaced the unrelated fresh-main OAuth fixture timeout tracked in Fix OAuth document-recovery E2E fixture ready-latch timeout #533; Firefox also produced the environment's known local timeouts.

Checklist

• npm test passes (the per-file coverage gate is non-negotiable)
• Tests added/updated in the same change as the code
• npm run build succeeds (single-file dist/sql.html)
• Layers kept honest: pure logic in src/core/, network in src/net/ (injected fetch), DOM in src/ui/
• No new runtime dependency (or it's a deliberate, justified addition — see CONTRIBUTING)
• README / CHANGELOG.md ([Unreleased]) updated if behavior or the deployed surface changed
• Reconciled affected tracked work (roadmap Roadmap to 1.0.0 #68, the issue body, ADR/CHANGELOG) if this change reshaped it