[2026-04] Backport auto POS button variant

[henryStelle]

Summary

• Backports Add auto to POS button variant type #4625 to the 2026-04 stable branch.
• Allows the public POS s-button variant type to explicitly accept auto in the individual and aggregate declarations.
• Updates the tracked 2026-04 POS generated docs JSON and adds an @shopify/ui-extensions patch changeset.
• Fixes https://github.com/shop/issues-retail/issues/33323.

Validation

• ✅ yarn build
• ✅ Focused TypeScript compile verifying auto in both individual and aggregate POS button variant types
• ✅ yarn type-check
• ✅ yarn lint
• ✅ Generated docs JSON parses and reports 'auto' | 'primary' | 'secondary'
• ℹ️ No dedicated POS button test suite is present on this branch.
• ⚠️ yarn docs:point-of-sale 2026-04 could not run with the provided symlinked dependencies because the generate-docs binary is absent; the tracked JSON was updated with the same semantic diff as Add auto to POS button variant type #4625.
• ⚠️ shopify validate components was unavailable because the shopify executable is not installed in this environment.

[henryStelle]

This PR explicitly backports #4625 to the 2026-04 stable branch.