Update notebook version metadata during normalize

[Rg9906]

Summary

Updates nbformat.validator.normalize() to synchronize notebook version metadata with the target version used during normalization.

Previously, normalizing a notebook to a newer schema version could introduce fields that are only valid in that version (for example, cell IDs introduced in notebook format 4.5) while leaving nbformat_minor unchanged. This could produce a notebook whose contents no longer matched the version declared in its metadata.

Changes

• Update normalize() to set:
  • nbformat = version
  • nbformat_minor = version_minor
• Add a regression test covering normalization from v4.0 to v4.5

Motivation

Addresses #328.

The issue discussion notes that when normalization upgrades a notebook to a newer schema version, the notebook metadata should also be updated to reflect the version used during normalization.

Testing

pytest