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/asyncrpcoperation_sweep.h

@@ -34,6 +34,10 @@ public:
 private:
     int targetHeight_;
     bool fromRPC_;
+    // Set by main_impl(): true iff there was nothing left to sweep this round.
+    // Read by main() to decide scheduler bookkeeping. Defaults false so an
+    // exception (which skips main_impl's assignment) is treated as "not done".
+    bool sweepComplete_ = false;
 
     bool main_impl();