Fix MySQL existence check and add router/store tests

- MySQL UPDATE returns 0 affected rows when values are unchanged, so
  detect existence via SELECT within the transaction instead.
- Add integration tests for PUT /locations/:id in createRouter.test.ts.
- Add changeset for catalog-react catalogApiMock update.

Signed-off-by: Fredrik Adelöw <freben@spotify.com>
Made-with: Cursor

.changeset/location-entity-ref-catalog-react.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-catalog-react': patch
+---
+
+Updated `catalogApiMock` to include the new `updateLocation` method stub, keeping it in sync with the `CatalogApi` interface.