fix(pool): diagnose empty pools, align catalog filter, and surface drop reasons

[maxmad64bis]

⚠️ base-red inherited: #12732

Summary

Empty auto/* pools now say why they're empty. When every candidate gets filtered out — paid-only backends under hidePaidModels, models locked on all connections, or free tiers without a documented hard stop under STRICT mode — the server logs which stage removed how many (open-sse/services/autoCombo/virtualFactory.ts), and combo health carries the reason on each provider entry (dropReason on quotaHealth.providers).

This also fixes three small defects found while wiring the diagnostics: uncapped recurring free quotas (real free access with no token cap, just rate limits) were treated as unknown instead of safe; an empty free-only import fell back to the stale synced model list instead of reporting empty; and the models listing used a looser paid check than routing (a payload isFree flag alone could keep a paid model listed).

The lockout filter (modelLockoutFilter.ts, wired in virtualFactory.ts) is always on for dispatch, flags off included; only paid (hidePaidModels) and STRICT (freeAccessPolicy=strict) are opt-in. The read-only candidate inspector skips it, so a locked account's row stays listed with reachable:false (#9133).

Related Issues

• Related to fix(models): stop treating provider-supplied free flags as trusted without a documented free tier #12744 (provider-supplied free flags no longer trusted without documentation — this aligns the paid predicate used there with routing)
• Related to fix(models): preserve free-model metadata from discovery #12763 (synced discovery preserving live free economics — the stale-fallback cut here is the import-side complement)

Validation

• Change type: provider / routing
• 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

Focused loop (all green, after rebasing onto the current tip): Vitest on paid-model-filter-6512 and the three strict-zero-cost-* suites (38 passed), node --import tsx/esm --test on the new suites plus auto-combo-candidates-locked-model-visible (42 passed), check:api-typecheck and typecheck:core within baseline, targeted eslint on all touched files (exit 0, no new suppressions). Full unit matrix, Vitest suites, coverage gate, and production build run in CI on this PR.

Tests Added Or Updated

• tests/unit/autoCombo/paid-model-filter-6512.test.ts (extended: 4 diagnosis tests)
• tests/unit/virtualFactory-pool-diagnosis.test.ts (new: paid/lockout/STRICT wiring)
• tests/unit/freeAccessQuota.test.ts (new: uncapped free-tier gate)
• tests/unit/sync-models-freeFilterEmpty.test.ts (rewritten to exercise the new resolveEffectiveAvailableModels helper, 3/3, neighbors green)
• tests/unit/catalog-hidePaid.test.ts (new: shared paid predicate)
• tests/unit/comboHealth-dropReason.test.ts (new: no-snapshot value assertion)
• tests/unit/auto-combo-candidates-locked-model-visible.test.ts (existing fix(providers): Only part of auto/* pool candidates were returned #9133 guard, green with the dispatch-only lockout)

Coverage Notes

Changes span open-sse/ and src/. Each production change is covered by its focused test above: the paid diagnosis (paidModelFilter.ts), the lockout filter (modelLockoutFilter.ts), the three aggregation logs (virtualFactory.ts), the uncapped-safe gate (freeAccessQuota.ts), the empty-import cut (sync-models/route.ts), the shared predicate (catalogPaidFilter.ts, used by catalog.ts), the STRICT exclusion count (strictZeroCostFilter.ts), and the health reason (comboHealth.ts, utilization.ts). No coverage movement measured locally; the CI coverage ratchet reports on this PR.

Reviewer Notes

The STRICT excluded N/M (no-hard-stop X) log counts with the filter's own verdict (countStrictExclusions, one pass over the pre-filter pool, no extra quota lookups since resolveFreeAccessState reads the same short-TTL cache), so a candidate the filter only narrows to fewer connections is never counted as excluded. The three stage logs share one warnPoolDrop helper, which keeps virtualFactory.ts under its 1200-line cap. decideHidePaid is stricter than the old listing closure: models previously kept listed on a bare isFree payload flag are now hidden unless the catalog backs them — intended, that's the routing behavior. FREE_TIERS.md gains one sentence documenting that STRICT mode only trusts hardStopGuaranteed entries. Merge after #12744: both touch the paid-model predicate in catalog.ts (decideHidePaid here follows the catalog-first rule from #12744).

• Changelog fragment included. 2nd freeFilterEmpty ternary (syncedModelsCount) kept inline with its explicit guard (its 3rd branch isn't a plain synced).