feat(core): restore Modal model discovery - #42851

AI code review — automated review for reference; please use your judgment.

Review of the Modal model discovery restore:

1. packages/core/src/plugin/provider/modal.ts (load, failure branches) — every failure path (fetch timeout, non-OK response, failed credential resolve, missing baseURL) assigns an empty models map, and the catalog transform then deletes all previously discovered Modal models — one flaky request or an expired key silently erases the user's model list instead of degrading gracefully — keep the last known-good models on transient failures (or flag them stale/disabled) and only clear when the connection is deliberately removed.
2. packages/core/src/plugin/provider/modal.ts (discover → build mapping) — a single malformed catalog entry (e.g. reasoning option values that Model.VariantID.make rejects) throws mid-map inside the shared tryPromise, aborting the entire sync and triggering issue 1's wipe — isolate per-item builds with their own catch, skip + log the offending entry so one bad row cannot discard the rest.
3. packages/core/src/plugin/provider/modal.ts (reasoning_options → variants) — variant ids are taken verbatim from API strings (value ?? "none") with no dedupe or charset validation — duplicated options yield duplicate variants and unexpected characters leak into downstream id handling — dedupe by resulting id and validate/sanitize before Model.VariantID.make.
4. packages/core/src/plugin/provider/modal.ts (catalog.transform removal loop) — removal iterates all models under the Modal provider and deletes any absent from the discovered set — statically configured/user-defined models for this provider would be pruned on every sync — track which ids were previously created by discovery and restrict removal to those.
5. packages/core/src/plugin/provider/modal.ts (templates cache) — templates memoizes the first catalog snapshot indefinitely, so later edits to base-model metadata (costs, limits, compatibility) never propagate into rebuilt models — re-read the catalog filter on each load or invalidate the cache on catalog-change events.
6. Test coverage — packages/core/test/plugin/provider-modal.test.ts covers the happy path well (mock server, auth header assertions), but none of the failure paths are tested — add cases for non-OK responses/timeouts asserting stale-model retention (item 1), malformed-entry skipping (item 2), and the no-credential clearing behavior.