feat(extensions): Querya Extension Driver Mutation Standard (#563)

[ZhuchkaTriplesix]

Summary

Implements the Querya Extension Driver Mutation Standard, defining and integrating data editing, staging, and mutation RPC capabilities across external database drivers.

Changes

1. ExtensionDriverCapabilities (lib/core/extensions/models/extension_driver_capabilities.dart):
   • Added supportsMutations and supportsBatchMutations capability flags with full JSON-RPC serialization / deserialization and equality checks.
2. ExtensionDriverSession Protocol (lib/core/extensions/extension_driver_session.dart):
   • Added getTableSchema method calling JSON-RPC db.getTableSchema to query table metadata (column types, nullability, primary keys).
   • Added mutate method calling JSON-RPC db.mutate for executing atomic batches of insert, update, and delete mutations.
3. ExtensionTableView Integration (lib/features/extensions/extension_table_view.dart):
   • Integrated with DataGridStagingBuffer when the underlying driver reports supportsMutations: true.
   • Wired _onApplyChanges to fetch schema and dispatch structured mutation batches via ExtensionDriverSession.instance.mutate.
4. Documentation (docs/tz-block-d-external-plugin.md):
   • Added Section 5 detailing the db.getTableSchema and db.mutate RPC request and response JSON schemas.
5. Tests:
   • Added unit & capability tests in test/core/extensions/extension_driver_session_test.dart and test/features/extensions/extension_table_view_test.dart.

Closes #563