Prepare Python 2026.12.0 release - #26287

Summary

Prepare the 2026.12.0 stable release using the merged release-PR automation.

• Change only root package.json's top-level version: 2026.11.0 -> 2026.13.0.
• Keep main on the next odd development minor, preserving the existing suffix-free version format.
• After approval and merge, the existing release workflow derives stable 2026.12.0 and creates release/2026.12.0 from the merged source. Its push can trigger Azure stable publishing; normal release approvals still apply.
• No lockfiles, dependencies, changelogs, workflows, tags, or publishing configuration are changed. No release branch has been created manually.

Prerequisite

Merge #26282 before merging this release PR. The read-only Release PR / Plan workflow is still absent from main. Once that workflow lands, update this PR branch to include it and verify a successful, non-skipped candidate Plan on the latest head. This PR intentionally remains version-only and does not duplicate that workflow change. Auto-merge is not enabled.

Validation

• Prettier and whitespace checks passed; the diff is one insertion and one deletion in root package.json only.
• The exact release/version Python scripts pinned by the consumer workflow at 4e0c4335bbf6b1a274673e22d9c39cd7efb93ccf validate this candidate locally.
• Isolated stable-manifest serialization passed without changing other JSON values; calculated branch: release/2026.12.0.
• PR creator repository write access verified. The exact target release ref was absent at preparation.

Issue tracking: routine release preparation; uses skip-issue-check and no-changelog rather than a separate product issue or changelog entry.

Version-only check policy

Uses the repository's existing skip package*.json label for this top-level version-only bump. No dependency or lockfile change is needed; changing a lockfile would make the release action classify this as an ordinary mixed-file PR. The sole impact label is no-changelog.