feat(txn): carry abort reasons as structured gRPC error details #9866

Follow-up to #9747.

#9747 reports why a transaction aborted as a category prefix on the codes.Aborted status message (conflict: Transaction has been aborted. Please retry. ...). Every client has to parse that prefix out of the message, and dgo (dgraph-io/dgo#288), dgraph4j (dgraph-io/dgraph4j#290), and pydgraph (dgraph-io/pydgraph#307) each carry their own copy of the parser and the category list.

This phase attaches the reason as structured detail using the gRPC rich error model: an errdetails.ErrorInfo on the status, with the category as Reason and a fixed Domain. Space in Metadata is reserved for the contended predicate once the server can name it.

• No proto change in dgo/protos/api; the detail rides on the status.
• Keep the message prefix in place so older clients and HTTP callers still work.
• Clients prefer the ErrorInfo when present and fall back to the prefix.

Open questions: the Domain string, and whether HTTP responses carry the same fields under extensions.