feat(txn): surface transaction abort reasons to clients

[rahst12]

Problem

When Dgraph aborts a transaction, Zero knows why — but the client never finds out.
Zero distinguishes at least three causes internally:

• a write-write conflict,
• a predicate move (the tablet relocated mid-transaction, or commits are blocked
  while a move is in flight), and
• a stale start-ts (the transaction predates the current Zero leader — i.e. a leader
  change, not a real conflict).

All three collapse into src.Aborted = true inside Server.commit()
(dgraph/cmd/zero/oracle.go), and by the time the error reaches the caller it is one
opaque string:

Transaction has been aborted. Please retry

Worse, the checkPreds path builds specific, human-readable messages for predicate
moves and then throws them away. With no discriminator, a client cannot make the one
decision the category enables: retry immediately (conflict) vs. back off
(predicate is moving).

Why we can't just name the conflicting predicate

For conflicts, you might expect the abort to name the contended predicate/UID. It can't —
not without extra bookkeeping. For every mutated edge, Alpha computes a conflict key in
GetConflictKey() (posting/list.go) as a one-way fingerprint:

farm.Fingerprint64(key) ^ uid

Only this uint64 is retained (conflicts map[uint64]struct{} in posting/oracle.go);
the predicate name and UID are dropped immediately. It is then base-36 encoded into
TxnContext.Keys and shipped to Zero, which detects a conflict purely by matching
fingerprints — it never sees a predicate name. Because the value is a fingerprint XORed
with a UID, it cannot be inverted to recover either input. Naming the predicate would
require remembering what was hashed (a per-transaction reverse map on Alpha), which is
larger scope. So this PR delivers the category, which needs no reverse mapping, and
leaves predicate/UID fidelity to follow-up work.

Fix

Thread a categorized reason out of Zero to the client. No change to conflict detection,
and no proto change — the reason rides on the existing codes.Aborted gRPC status as
"<reason>: <detail>".

• dgraph/cmd/zero/oracle.go — commit() tags each abort with a category
  (conflict, stale-startts, predicate-move); predicate-move now reuses the existing
  checkPreds messages instead of swallowing them.
• worker/mutation.go — CommitOverNetwork forwards the reason and records the
  abort metric, instead of flattening it to the reasonless dgo.ErrAborted.
• edgraph/server.go — both commit paths (CommitNow and explicit CommitOrAbort)
  pass the reason through and set Aborted.

Backward compatible: the existing message is preserved and the status code stays
codes.Aborted, so existing client retry logic is unaffected. This phase is
category-only; naming the contended predicate/UID is planned follow-up.

Future work

This PR is deliberately scoped to the category only — the cheapest, lowest-risk slice
that is immediately useful to clients — because it needs no reverse mapping, no new proto
fields, and no change to conflict detection. All correctness risk stays in what we
report, not what we decide. Richer fidelity is layered, additive work that can land
independently:

• Name the contended predicate(s). Retain a small per-transaction reverse map on
  Alpha (fingerprint → {predicate, uid, kind}), populated where the conflict key is
  already computed. The aborting transaction's own Alpha still holds this metadata, so
  Zero need only echo back which fingerprint matched — no hash inversion. The cost lands
  only on the abort path. Target the single-request CommitNow path first.
• Structured, machine-readable detail. Carry category + predicate + uid/token via the
  gRPC rich-error model (ErrorInfo details) so clients get typed fields instead of
  parsing a message string. Still no proto change.
• First-class API field. Eventually promote the reason/conflict detail to a real
  TxnContext field in the dgo proto, giving uniform structured access across all
  language clients — at the cost of a coordinated cross-repo release.
• Refine the predicate-move category. Today all checkPreds failures report as
  predicate-move, but some are not moves (e.g. a malformed/internal predicate key, or a
  predicate not currently served by any group). A later phase can split these into honest
  categories (predicate-unavailable, internal) and revisit retryability — an
  internal/malformed abort is not retryable, unlike a move.

Checklist

• The PR title follows the
  Conventional Commits syntax, leading
  with fix:, feat:, chore:, ci:, etc.
• Code compiles correctly and linting (via trunk) passes locally
• Tests added for new functionality, or regression tests for bug fixes added as applicable
• Refer to dgraph4j, pydgraph, and dgraph-docs as corresponding updates are made.