[2026-07] Backport auto POS button variant

[henryStelle]

Summary

Backports #4625 to the 2026-07 stable branch.

• allow the public POS s-button variant type to explicitly accept auto
• keep the individual and aggregate component declarations consistent
• update the tracked 2026-07 generated POS docs JSON
• add a patch changeset for @shopify/ui-extensions

Fixes https://github.com/shop/issues-retail/issues/33323

Validation

• ✅ yarn build
• ✅ focused TypeScript compile proving auto is accepted by both individual and aggregate ButtonJSXProps
• ✅ yarn type-check
• ✅ yarn lint
• ✅ generated docs JSON parses successfully
• ⚠️ yarn test -- packages/ui-extensions/: environment/dependency failures in unrelated checkout Preact suites (react-test-renderer is missing from the shared symlinked node_modules, plus one TSX transform failure); 2 suites / 19 tests passed, and this type-only POS change has no relevant POS runtime test suite
• ⚠️ yarn docs:point-of-sale 2026-07: the shared symlinked node_modules does not provide the generate-docs command; the tracked JSON update matches the source backport patch
• ⚠️ shopify validate components: Shopify CLI is not installed in this environment

[henryStelle]

This PR explicitly backports #4625 to the 2026-07 stable branch.

[github-actions]

This PR targets a stable release branch (2026-07). Once merged, the change typically also needs to be forward-ported to 2026-10-rc so it ships in the next release.

When you open the forward-port PR, include a line like this in its body so the needs-rc-port label gets removed automatically when that PR merges:

Forward-port of #4626

Accepted formats (comma-separated for multiple):

• #4626
• GH-4626
• 4626
• https://github.com/Shopify/ui-extensions/pull/4626

If a forward-port isn't needed (e.g., the change is stable-only), you can remove the needs-rc-port label manually.