[codex] Allow window transforms without fields

[EltonChang1]

Summary

Fixes #585.

Vega treats the fields property on window transforms as optional. VegaFusion's parser currently requires it, so valid specs such as a row_number window transform without fields fail during deserialization.

This change:

• allows WindowTransformSpec.fields to deserialize when omitted
• normalizes omitted/empty fields to one empty field per window op when building the internal transform proto
• adds focused coverage for parsing and evaluating a row_number window transform without fields

Validation

• git diff --check

I could not run the Rust test suite locally because this host has no cargo, rustc, or pixi on PATH, and Docker is installed but its daemon is not running. The targeted tests added are:

• omitted_fields_default_to_one_empty_field_per_op
• test_window_row_number_without_fields

AI disclosure

This draft PR was prepared with assistance from OpenAI Codex/ChatGPT. The code changes, tests, PR description, and validation summary were AI-assisted and reviewed in the local checkout before submission.