wallet: fix sweep-scheduler wedge and unsynchronized driver mutation

The zaddr-sweep op cleared fSweepRunning/nextSweep only on the sweepComplete
success path (inside main_impl), so a cancelled or throwing sweep left
fSweepRunning stuck true. Since fSweepRunning now also gates consolidation and
the default-on autoshield, a persistently failing sweep (e.g. a corrupt-witness
note) would wedge all three background ops for the session.

Move the scheduler bookkeeping into main() so it runs on every terminal state
(success/failure/exception/cancel), guarded by op id. Preserve the intended
"keep draining every block until swept" model: on a successful-but-incomplete
round the flag stays set and nextSweep is not advanced; on completion OR on
failure/exception the flag is released and nextSweep backs off one interval, so
a failing sweep no longer retries every block or wedges the other ops.

Also fix RunSaplingSweep to take cs_wallet itself (was AssertLockHeld, a no-op
in release builds) since ChainTip does not hold it there and the driver mutates
scheduler state + enqueues -- matching RunSaplingConsolidation and
RunAutoShieldCoinbase.

sweepComplete is recorded via a new member; saplingSweepOperationId made public.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/wallet/wallet.h

@@ -786,7 +786,6 @@ private:
     std::vector<CTransaction> pendingSaplingConsolidationTxs;
 
     std::vector<CTransaction> pendingSaplingSweepTxs;
-    AsyncRPCOperationId saplingSweepOperationId;
 
     void AddToTransparentSpends(const COutPoint& outpoint, const uint256& wtxid);
     void AddToSaplingSpends(const uint256& nullifier, const uint256& wtxid);
@@ -831,6 +830,9 @@ public:
     int rescanStartHeight             = 0;
 
     int nextSweep                     = 0;
+    // Id of the in-flight sweep op; read by the op to confirm it is still the
+    // current one before mutating scheduler state.
+    AsyncRPCOperationId saplingSweepOperationId;
     int amountSwept                   = 0;
     int amountConsolidated            = 0;
     int sweepInterval                 = 10;