feat(mongodb): visual Index Manager (#1028)

[ZhuchkaTriplesix]

Closes #1028

Summary of Changes

• MongoDB Service:
  • Added getIndexesWithStats retrieving collection indexes and augmenting them with on-disk index sizes from collStats.
  • Added createIndex supporting composite key directions (1, -1, text, 2dsphere), custom index name, unique, sparse, and expireAfterSeconds (TTL).
  • Added dropIndex with primary index guard (rejects dropping _id_).
• Data Models:
  • Implemented MongoIndexInfo with metadata extraction, formatted keys, type badges (Primary, TTL, Text, Geospatial, Compound, Single Field), and readable size formatting.
• UI:
  • Added MongoIndexesView with index list, badge indicators, on-disk size stats, refresh capability, and protected delete actions.
  • Added MongoCreateIndexDialog with interactive compound index builder, direction selectors, index options (Unique, Sparse), optional TTL input, and validation.
  • Integrated "Indexes" navigation button into MongoDocumentsView and breadcrumb routing in MongoExplorerView.
• Testing:
  • Added unit tests in test/features/mongodb/mongo_index_info_test.dart.
  • Added widget tests in test/features/mongodb/mongo_indexes_view_test.dart covering index listing, primary key delete protection, drop confirmation, and index creation flows.