fix(qwp): attach the throwable to the drainer's retryable-setup warning

The catch (Exception) setup path in BackgroundDrainer logged only
t.getMessage() and stored it as lastErrorMessage. For a JVM-raised NPE
(getMessage() == null on JDK 8, the release target) that produced
'drainer setup temporarily unavailable for slot ...: null' with no
exception class and no stack trace -- and since this path deliberately
leaves no .failed sentinel, a deterministic setup bug is retried on
every orphan scan while emitting that same contentless line forever.
The outer catches in the same method already attach the throwable
(LOG.error(..., slotPath, msg, t)); only this inner retryable catch
dropped it.

Log the throwable (SLF4J attaches the stack trace) and carry
t.toString() -- class plus message -- into lastErrorMessage so the
telemetry surface shows 'java.lang.NullPointerException' instead of
'null'. Diagnostics only; the retry-not-quarantine policy is unchanged.

sf package: 391 tests green.

Author: bluestreak01

core/src/main/java/io/questdb/client/cutlass/qwp/client/sf/cursor/BackgroundDrainer.java

@@ -582,9 +582,15 @@ public void run() {
                 // unlock. Leave no .failed sentinel for the next orphan scan.
                 // Error deliberately escapes to the outer Error path: it is
                 // observable after teardown but cannot quarantine intact data.
-                String msg = t.getMessage();
+                // This path retries on every orphan scan, so the log line is
+                // the ONLY diagnostic a deterministic bug (e.g. an unexpected
+                // NPE, whose getMessage() is null) ever produces: attach the
+                // throwable for the stack trace and carry class+message into
+                // the telemetry surface, mirroring the outer setup-failure
+                // catch below.
+                String msg = t.toString();
                 LOG.warn("drainer setup temporarily unavailable for slot {}: {}",
-                        slotPath, msg);
+                        slotPath, msg, t);
                 lastErrorMessage = msg;
                 outcome = DrainOutcome.FAILED;
                 return;