feat(mongodb): Aggregation Pipeline Builder & Runner (#1027)

[ZhuchkaTriplesix]

Closes #1027

Summary

• MongoService: Stream documents via aggregateToStream(pipe) with fallback to cursor firstBatch / result array extraction to return raw result documents.
• MongoAggregationStage: Data model for stages with operators ($match, $group, $project, $sort, $limit, $skip, $lookup, $unwind, $addFields, $count, etc.), templates, JSON/EJSON validator, and code exporter (MongoAggregationExporter) for mongosh, Node.js, and Python.
• MongoAggregationWorkspace: Interactive visual & text stage builder supporting:
  • Add, remove, reorder (move up/down), duplicate, and enable/disable toggle for stages
  • Operator selection via QueryaDropdown with syntax validation and inline error banner
  • "Run up to here" per-stage execution and full pipeline execution with stopwatch execution metrics (docs count and ms)
  • Results preview in both Table Grid and JSON View
  • Export dialog with clipboard copy for mongosh, Node.js, and Python
• Navigation Integration:
  • Added Aggregation button to MongoDocumentsView toolbar
  • Added _Level.aggregation breadcrumb support to MongoExplorerView
• Tests:
  • Unit tests for stage validation, parsing, pipeline building, and code generation (test/features/mongodb/mongo_aggregation_stage_test.dart)
  • Full widget test suite for MongoAggregationWorkspace (test/features/mongodb/mongo_aggregation_workspace_test.dart)
  • All tests and architectural guards passed.