Fix client-visible transaction status flag desync after DDL implicit commit

[flux423]

Problem

ServerInTransaction (SERVER_STATUS_IN_TRANS) is reported based solely on
ctx.GetTransaction() != nil in server/handler.go::setConnStatusFlags().
That's also true for the engine's implicit, per-statement transaction used
for ordinary autocommit statements — not just for an explicit client
BEGIN.

Combined with a second gap — TransactionCommittingIter.Close() in
sql/rowexec/transaction_iters.go clears the transaction on an implicit
commit (e.g. a DDL statement issued mid-explicit-transaction) but never
clears ctx.GetIgnoreAutoCommit(), unlike the explicit COMMIT/ROLLBACK
handlers in sql/rowexec/transaction.go, which do — the very next ordinary
autocommit statement issued right after such a DDL gets misreported to the
client as still being inside a transaction.

This desyncs any MySQL client whose own transaction bookkeeping mirrors the
server status flag — notably PHP's PDO_MySQL, whose PDO::inTransaction()
and the guard inside PDO::beginTransaction() read exactly this flag. The
client correctly believes no transaction is open (it never issued a
matching COMMIT/ROLLBACK for anything), then refuses its own next
explicit BEGIN client-side, before any query reaches the wire, with
"There is already an active transaction".

Real-world repro

Found via Drupal core's drush site:install standard: an unmodified Drupal
installation, using Drupal's stock mysql PDO driver (no custom driver, no
Drupal patches), failed late in install (the site-configure step, after all
~30 standard-profile modules had already installed their schema
successfully) with exactly this PDOException. The identical install
completes cleanly against MySQL 8.0.46 with the same Drupal codebase and
command.

Minimal reproduction

$pdo->beginTransaction();
$pdo->exec('CREATE TABLE t (id INT PRIMARY KEY)'); // implicit commit
$pdo->exec('INSERT INTO t VALUES (1)');             // ordinary autocommit statement

var_dump($pdo->inTransaction());
// MySQL 8.0:                false (correct)
// Dolt (before this fix):   true  (incorrect - no BEGIN was issued for this)

$pdo->beginTransaction();
// MySQL 8.0:                succeeds
// Dolt (before this fix):   PDOException: There is already an active transaction

Fix

• sql/rowexec/transaction_iters.go: TransactionCommittingIter.Close()
  now calls ctx.SetIgnoreAutoCommit(false) when committing an implicit
  transaction, mirroring the explicit COMMIT/ROLLBACK handlers.
• server/handler.go: setConnStatusFlags() now additionally requires
  ctx.GetIgnoreAutoCommit() before reporting ServerInTransaction, as
  defense-in-depth so an implicit per-statement transaction can never set
  the client-visible flag even if a future code path reintroduces a similar
  gap.

Testing

• Added TestSetConnStatusFlagsInTransaction (server/handler_test.go),
  covering no-transaction, implicit-per-statement-transaction, and
  explicit-client-transaction cases.
• go test ./server/... ./sql/rowexec/...: pass.
• go test ./enginetest/... -run "Transaction|Commit|Autocommit": pass,
  including the create_table_queries_are_implicitly_committed and related
  DDL-implicit-commit suites.
• End-to-end: rebuilt Dolt against this branch and re-ran
  drush site:install standard against an unmodified Drupal 11.4-dev
  core — install now completes successfully (previously failed at the same
  point on unpatched Dolt 2.1.10, 2.3.0, and main).

Related, separately-filed issues from the same investigation (distinct root
causes, not fixed by this PR): #3706 (constraint-violation SQLSTATE
mapping), #3707 (SHOW INDEX ... WHERE key_name filter ignored).