🔄 fix: Reindex MeiliSearch Documents After Indexed-Projection Changes (#15484)

* 🔄 fix: Reindex MeiliSearch Documents After Indexed-Projection Changes

- Introduce _meiliIndexSchemaVersion schema field and MEILI_INDEX_SCHEMA_VERSION constant
- Stamp schema version on single-document and batch index writes
- Query unindexed and stale-version documents in syncWithMeili
- Count stale documents in pendingIndexing in getSyncProgress
- Add spec coverage for reindexing stale-version documents

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* 🔄 test: use the registered Meili conversation model

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* 🔄 style: satisfy Prettier in Meili indexing progress counts

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Danny Avila <danny@librechat.ai>