[IcebergIO] Fix stale openWriters count in RecordWriterManager (#39157)

RecordWriterManager uses a Guava Cache with expireAfterAccess(1 min)
for writer eviction. However, Guava Cache eviction is lazy — expired
entries are only removed on subsequent access or explicit cleanUp().
The write() method checks `openWriters >= maxNumWriters` without first
calling cleanUp(), so expired writers remain in the cache and
openWriters stays stale.

With DEFAULT_MAX_WRITERS_PER_BUNDLE = 20 and more than 20 partitions,
this causes 100% false spill — every record for the 21st+ partition
is rejected as if the writer pool is full.

Add writers.cleanUp() before the capacity check to evict expired
entries and update openWriters accurately.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: atognolas

sdks/java/io/iceberg/src/main/java/org/apache/beam/sdk/io/iceberg/RecordWriterManager.java

@@ -163,7 +163,10 @@ boolean write(Record record) {
 
       @Nullable RecordWriter writer = writers.getIfPresent(routingPartitionKey);
       if (writer == null && openWriters >= maxNumWriters) {
-        return false;
+        writers.cleanUp();
+        if (openWriters >= maxNumWriters) {
+          return false;
+        }
       }
       writer = fetchWriterForPartition(routingPartitionKey, writer);
       writer.write(record);