test(grid): add end-to-end integration tests for data grid staging, DML generation, and filtering pipelines (#578)

[ZhuchkaTriplesix]

Summary

• Implements complete End-to-End integration test suite for the Interactive Data Grid subsystem:
  • Staging mutations (cell edits, row insertions, row deletions).
  • Multi-dialect DML generation (Postgres, MySQL, SQLite) with atomic transaction wrapping (BEGIN, COMMIT).
  • Table name & schema extraction from SELECT statements via SqlTableTargetExtractor.
  • Filter evaluation -> Selection calculation -> Hierarchical Groupings Pivot pipeline.
  • Reset & clear operations.

Closes #578

Verification

• flutter analyze lib/features/main_screen/ passed with 0 issues.
• flutter test test/features/main_screen/data_grid_e2e_integration_test.dart passed all 3 integration test scenarios.