[matthewmcneely]

Description

This PR surfaces the server's transaction abort reason in dgo, matching what dgraph-io/dgraph4j#290 and dgraph-io/pydgraph#307 do for the Java and Python clients.

dgraph-io/dgraph#9747 has Zero report why it aborted a transaction as a "<code>: <detail>" prefix on the gRPC Aborted status (conflict, predicate-move, or stale-startts). Today Txn flattens every Aborted status to the bare dgo.ErrAborted sentinel, so callers can't tell a write-write conflict (retry now) from a predicate move (back off first).

Changes

• New AbortReason type with AbortReasonConflict, AbortReasonPredicateMove, AbortReasonStaleStartTs, and AbortReasonUnknown.
• New *AbortedError{Reason, Message}, returned by Txn.Commit and by the mutation paths of Txn.Mutate/Txn.Do in place of ErrAborted. Error() returns the server's message verbatim, same as getMessage() in dgraph4j and str(e) in pydgraph.
• AbortedError implements GRPCStatus(), so status.Code(err) reports codes.Aborted instead of codes.Unknown.
• ParseAbortReason(msg) is exported for callers using the raw api.DgraphClient. Parsing follows the other two clients: text before the first colon, trimmed and case-insensitive. Anything unrecognized maps to AbortReasonUnknown, which covers every message from a server without #9747 as well as causes the server deliberately leaves uncategorized.
• README: the commit example now uses errors.Is (it also had a stray y.ErrAborted typo), plus a short errors.As example.

var ae *dgo.AbortedError
if errors.As(err, &ae) && ae.Reason == dgo.AbortReasonPredicateMove {
  // back off before retrying
}

Compatibility

AbortedError unwraps to ErrAborted, so errors.Is(err, dgo.ErrAborted) keeps working, and every abort message from the server still contains "Transaction has been aborted. Please retry", so substring checks on err.Error() keep matching too. A direct err == dgo.ErrAborted comparison no longer matches. The Java and Python clients could hang the reason off their existing exception type, but Go can't attach data to a sentinel value, so this is the trade. It should get a call-out in the release notes.

The dgraph repo had a handful of == dgo.ErrAborted comparisons in the live loader and systests. Those are switched to errors.Is in #9747, so bumping dgraph to this dgo is safe.

Testing

• abort_test.go: unit tests for prefix parsing (including colons inside the detail, unknown categories, and pre-#9747 messages) and for errors.Is/errors.As through a wrapped error.
• TestTxnErrAborted now actually checks the error. The old require.Error(t, err, dgo.ErrAborted, ...) only checked for non-nil, since the sentinel landed in msgAndArgs. It asserts ErrorIs, ErrorAs, and a reason of conflict or unknown, so it passes against dgraph main before and after #9747 merges.
• Ran the full suite against a local cluster built from the #9747 branch; the conflict test reports AbortReasonConflict with the server's full conflict message.

Checklist

• Code compiles correctly and linting passes locally
• Tests added for new functionality, or regression tests for bug fixes added as applicable
• Server side: feat(txn): surface transaction abort reasons to clients dgraph#9747

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