* fix: add authenticate flag to registerCustomer mutation
Adds an optional `authenticate` boolean input to the registerCustomer
mutation. When true, the mutation sets the current user and reinitializes
the session token for the newly registered customer. Defaults to false
to prevent nonce verification failures in contexts like GraphiQL that
send a nonce with the request.
Resolves#464
* test: add authenticate flag to existing register customer tests
The existing wpunit tests for registerCustomer rely on the user being
authenticated after registration to assert on customer.databaseId and
viewer.userId. Now that authenticate defaults to false, the tests must
explicitly pass authenticate: true.
* test: add authenticate flag to testCustomerMutationsWithMeta
* fix: use explicit billing emails in CustomerQueriesTest assertions
Use wildcard index matching with specific email values instead of
hardcoded node indices with NOT_NULL checks. This prevents failures
when other users in the DB shift the node positions.
