AI Assistant: validate filter field existence for filter command

[anna-shakhova]

No description provided.

[Copilot]

Pull request overview

This PR tightens validation for the Grid AI Assistant’s filterValue command by ensuring every basic filter expression targets an existing grid column, preventing invalid filter sync behavior.

Changes:

• Validate expr.field by resolving the corresponding column via columnOption, and fail the command when the column does not exist.
• Use the resolved column’s dataType when normalizing filter values (instead of querying dataType directly).
• Add Jest coverage for unknown fields and for filtering on hidden-but-existing columns.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
packages/devextreme/js/__internal/grids/grid_core/ai_assistant/commands/filtering.ts	Adds column existence validation for basic filter nodes and updates the command description to require existing columns.
packages/devextreme/js/__internal/grids/grid_core/ai_assistant/commands/tests/filtering.test.ts	Adds tests ensuring unknown fields fail without applying filterValue, and hidden columns still allow filtering.