feat: surface transaction abort reason on TxnConflictException

[rahst12]

Problem

When Dgraph aborts a transaction, the Java client collapses every cause into a single
opaque exception:

io.dgraph.TxnConflictException: Transaction has been aborted. Please retry

TxnConflictException exposes no way to tell why the transaction aborted, so an
application cannot distinguish the cases that warrant different responses:

• a write-write conflict — retry immediately with a fresh transaction;
• a predicate move — a predicate is relocating between groups and commits on it are
  temporarily blocked, so back off and retry once the move completes;
• a stale start-ts — the transaction predates the current Zero leader (a leader
  change); retry with a fresh transaction.

The server now reports the category (see the companion dgraph PR), encoding it as a
"<code>: <detail>" prefix on the gRPC ABORTED status description. Without client
support, that information is only visible by string-scraping getMessage().

Example

Before, every abort looked the same — there was no way to branch on the cause:

catch (TxnConflictException e) {
  retryWithNewTxn(); // the only option, regardless of why it aborted
}

Now the category is available via getReason(), so callers can respond appropriately:

try {
  txn.mutate(mutation); // or txn.commit()
} catch (TxnConflictException e) {
  switch (e.getReason()) {
    case CONFLICT:       // write-write conflict — retry now with a fresh txn
    case STALE_STARTTS:  // leader change — retry with a fresh txn
      retryWithNewTxn();
      break;
    case PREDICATE_MOVE: // predicate relocating — back off, then retry
      backoffAndRetry();
      break;
    case UNKNOWN:        // older server, or unrecognized reason
    default:
      log.warn("Txn aborted: {}", e.getMessage()); // full text still available
      retryWithNewTxn();
  }
}

Fix

PreReq: dgraph-io/dgraph#9747

Expose the category as a typed value on TxnConflictException, parsed from the status
description the server already sends.

• AbortReason enum — CONFLICT, PREDICATE_MOVE, STALE_STARTTS, UNKNOWN.
• TxnConflictException.getReason() — parses the "<code>: <detail>" prefix off the
  gRPC status and maps it to an AbortReason. The full human-readable te
  available via getMessage().

Backward compatible by design:

• Against an older server that reports no reason, getReason() returns
  UNKNOWN, so callers degrade gracefully.
• getMessage() is unchanged, isRetryable() still returns true, and the exception
  type/hierarchy is unchanged — existing catch blocks and retry loops are unaffected.

Tests

• AbortReasonTest — unit tests feeding synthetic gRPC ABORTED sta
  Exceptions.translate, asserting each category parses correctly and that an absent or
  prefix-less description degrades to UNKNOWN.
• AbortReasonLiveTest — cross-language end-to-end test that drives a real
  (locally patched) Dgraph cluster, forces each abort category, and asserts it propagates
  all the way to getReason(). Skips gracefully when the cluster prerequisites
  (multi-group for predicate-move, a Zero-restart hook for stale-startts)
  configured. A docker-compose.abort-reason.yml is included to stand up that cluster.

Future work

This change surfaces the category only — the slice the server provide
server enriches aborts with the contended predicate and UID/token (planned dgraph
follow-up via the gRPC rich-error model, then a first-class TxnContext field), this
client can add typed accessors such as getConflictPredicates() and getConflicts()
without breaking the getReason() surface introduced here. A later phase may also expand
AbortReason (e.g. splitting non-move predicate-move cases into PREDICATE_UNAVAILABLE
/ INTERNAL) and revisit isRetryable() for the non-retryable internal
codes already map to UNKNOWN, so adding values stays backward compatible.

Checklist

• The PR title follows the
  [Conventional Commits](https://www.conventionalcommits.org/en/v1.0.
  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 fixe
• This PR is required first: feat(txn): surface transaction abort reasons to clients dgraph#9747

[CLAassistant]

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