fix(db): ignore non-finite rate_limited_until writes, preserve null clear

[maxmad64bis]

⚠️ base-red inherited: #12732

Summary

Rate-limit cooldowns are timestamps in the database, and until now any value got written — including ones that make no sense, like NaN, or ones already in the past. A stale write could leave a connection looking locked (or unlocked) at the wrong time. Now only future timestamps are stored; anything expired or invalid is quietly skipped, and clearing a cooldown works exactly as before.

Related Issues

• Related to feat(providers): advertise free-tier capability in provider plugin manifest #12786 and fix(quota): dedup concurrent getSaturation misses with singleflight #12787 (sibling changes from the same batch; disjoint files, no dependency)

Validation

• Change type: DB
• Focused tests and category gates from the golden path
• npm run lint
• Reconciled with the current active release base; focused checks rerun afterward
• Production-code changes include a new or updated automated test in this PR

Tests Added Or Updated

• tests/unit/db-rate-limit-guard.test.ts (new) — 6 cases: NaN ignored, ±Infinity ignored, past/0 ignored, future writes through, null clear preserved, expired write doesn't overwrite a live row (6/6 pass; 3 fail before the guard as designed)
• Neighbors green: every suite under tests/unit that calls setConnectionRateLimitUntil — db-providers-crud, db-providers-split, antigravity-429-quota-cooldown, persist-429-cooldown-account-fallback, cooldown-epoch-string-3954, mark-account-unavailable-numeric-epoch-guard (82/82 pass)

Coverage Notes

• Changes src/lib/db/providers/rateLimit.ts (one fused guard + comment at the head of setConnectionRateLimitUntil; readers, markConnectionRateLimitedUntil, and clearConnectionRateLimit untouched). Covered by the new test file; no new branches beyond the guard itself.

Reviewer Notes

• One deliberate call: past timestamps are noops rather than implicit clears, so an expired write can never overwrite a live future row — the only clear path stays null via clearConnectionRateLimit. If any caller relied on writing the past to "unlock" a row, that pattern now needs an explicit clear (no such caller found in the test suites above).
• No migrations, no flags, no routing changes. Single commit.