fix(datagrid): persist column widths in columns config

[LarryHu0217]

Description

Column widths are column-level style state, so this change moves their persistence to ViewerConfig.columns_config.

• advertise column_size_override through the Datagrid column config schema
• publish drag-resized widths to columns_config while preserving other column styles
• restore and reapply widths across the first Datagrid draw
• stop writing widths to plugin_config
• disable width persistence when split_by is populated
• add regression coverage for resize publication, first restore, and split-by behavior

Closes #3163.

Validation

• Prettier check on all eight changed files
• ESLint on all eight changed files
• focused schema assertion for normal and split-by configurations
• focused pre-initialization width-cache assertion
• scoped git diff --check

The full Playwright test was not run locally because this checkout does not have the generated client/viewer WASM artifacts; the regression tests are included for CI.

AI assistance

I used OpenAI Codex to help inspect the codebase, implement the change, draft the regression tests, and run the local checks. I reviewed the final diff and validation results myself.