feat(config): add atomic saves and revision rollback

[romy651]

Summary

Adds atomic, versioned configuration saves and revision rollback to the LibreChat Admin Panel.

• send base saves, resets, imports, deletions, and restores through LibreChat's atomic mutation endpoint with expectedVersion
• reject stale saves instead of silently overwriting a newer administrator's change
• add configuration revision history and restore controls
• connect revision handling to the same MongoDB database used by LibreChat
• reconstruct indexed edits as complete array writes so dotted paths cannot turn arrays into numeric-key objects
• merge custom endpoints by name and preserve stored secrets without copying YAML-only credentials into MongoDB
• keep explicit secret and credential-container clears authoritative
• validate indexed values, bounds, reset paths, ambiguous unions, and unsafe field paths before submitting a mutation
• enforce the backend's combined 100-operation mutation limit

Backend dependency: danny-avila/LibreChat#15458

The backend must be merged and deployed before, or in the same release train as, this PR.

Change Type

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Translation update

Testing

• bun run lint
• bun run test — 28 test files and 920 tests
• bun run build

Coverage includes atomic save payloads, revision lifecycle and restore behavior, CAS conflicts, indexed-array reconstruction, custom endpoint keyed merges, secret preservation and explicit clearing, mutation limits, unsafe path rejection, and reset-path validation.

Test Configuration

• Bun 1.3.14
• macOS
• server-function tests use mocked LibreChat API, session, and capability boundaries
• revision lifecycle tests use the in-memory revision store

Checklist

• My code adheres to this project's style guidelines
• I have performed a self-review of my code
• I have commented in complex areas of the code
• I have made pertinent README and environment-example updates
• My changes do not introduce new warnings
• I have written tests demonstrating that my changes are effective
• Local unit tests pass with my changes