feat: surface transaction abort reason on AbortedError

[rahst12]

Problem

When Dgraph aborts a transaction, pydgraph collapses every cause into a single opaque
exception:

pydgraph.errors.AbortedError: Transaction has been aborted. Please retry

AbortedError 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. Previously pydgraph discarded
even the server's message — it raised a bare AbortedError with a hardcoded default
string — so that information never reached the caller at all.

Fix

Surface the category as a typed attribute on AbortedError, parsed from the message the
server already sends.

• AbortReason enum — CONFLICT, PREDICATE_MOVE, STALE_STARTTS, UNKNOWN
  (exported from the top-level pydgraph package).
• AbortedError.reason — the parsed AbortReason, derived via parse_abort_reason()
  from the "<code>: <detail>" prefix. The full server text remains available via
  str(error).
• Plumbing — the sync and async commit/mutate paths (txn.py, async_txn.py) now
  pass the server's message into AbortedError instead of raising a reasonless default;
  util.abort_error_message() extracts it from the gRPC error (preferring
  error.details(), falling back to str(error)).

Backward compatible by design:

• Against an older server that reports no categorized prefix, reason is
  AbortReason.UNKNOWN, so callers degrade gracefully.
• The exception type is unchanged and it remains raised from the same paths — existing
  except AbortedError / retry logic is unaffected.

Tests

• tests/test_abort_reason.py — unit tests for parse_abort_reason() and
  AbortedError.reason: each category parses correctly, and an empty or prefix-less
  message degrades to UNKNOWN.
• tests/test_abort_reason_live.py — end-to-end test that drives a real (locally
  patched) Dgraph cluster, forces each abort category, and asserts it propagates to
  AbortedError.reason. Skips gracefully when cluster prerequisites are unavailable.
  tests/docker-compose.multigroup.yml stands up the multi-group cluster needed for the
  predicate-move case.

Future work

This change surfaces the category only — the slice the server provides today. Once the
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 (e.g. conflict_predicates) without breaking the reason
attribute introduced here. A later phase may also expand AbortReason (e.g. splitting
non-move predicate-move cases into PREDICATE_UNAVAILABLE / INTERNAL); unknown codes
already map to UNKNOWN, so adding values stays backward compatible.

Example

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

except pydgraph.AbortedError:
    retry_with_new_txn()  # the only option, regardless of why it aborted

Now the category is available via error.reason:

try:
    txn.mutate(set_obj=data)  # or txn.commit()
except pydgraph.AbortedError as error:
    if error.reason is pydgraph.AbortReason.PREDICATE_MOVE:
        backoff_and_retry()   # predicate relocating — back off, then retry
    elif error.reason is pydgraph.AbortReason.UNKNOWN:
        logging.warning("Txn aborted: %s", error)  # full text still available
        retry_with_new_txn()
    else:  # CONFLICT or STALE_STARTTS — retry with a fresh txn
        retry_with_new_txn()

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
• Requires PR feat(txn): surface transaction abort reasons to clients dgraph#9747

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