[kylecannon]

Summary

Since SQS fair queues, standard queues take a MessageGroupId and hand it back on receive. rustack still rejected it as FIFO-only, so a client using fair queues failed on every send.

• Standard messages now keep their group id, and it comes back as the MessageGroupId attribute on receive.
• The id is validated the same way on both queue types (1-128 letters, digits and punctuation, via validate_message_group_id). FIFO queues weren't checking it before either.
• The dead-letter path used "has a group id" to mean "is FIFO", which would have kept grouped standard messages from ever reaching their DLQ. The source queue now passes its type along with the transfer (source_is_fifo on QueueCommand::Transfer).
• The sqs-compat test that expected the old rejection is flipped. MessageDeduplicationId is still rejected on standard queues.

Test plan

• Unit tests for keeping the group id on receive, the format check on both queue types, and dead-lettering a grouped standard message; an integration test for sending with a group id; sqs-compat tests for an accepted and an invalid id.
• cargo test -p rustack-sqs-core and the SQS integration tests pass; sqs-compat passes with the flipped test.