[Docs][Core] Add Javadoc to MultiTableWriterRunnable

Sub-issue of #10533

Scope

MultiTableWriterRunnable is the async worker that drains a BlockingQueue<SeaTunnelRow> and dispatches rows to per-table SinkWriter instances. It has no method-level Javadoc despite containing subtle concurrency and schema-evolution logic.

File

seatunnel-api/src/main/java/org/apache/seatunnel/api/sink/multitablesink/MultiTableWriterRunnable.java

Methods to document

Method	Line	What to explain
Constructor	37	tableWriterMap keyed by tableIdentifier string; queue ownership (each runnable owns exactly one BlockingQueue)
run()	45	Infinite loop draining the queue; row.getArity() == 0 as control signal for schema-evolution events; synchronized(this) block ensures snapshotState cannot run concurrently with an active write
getThrowable()	91	Returns first unhandled exception from the run loop; null means the worker is healthy; checked by MultiTableSinkWriter.subSinkErrorCheck()
getCurrentTableId()	95	Diagnostic field: the tableId of the row currently being written; may be null or stale between rows

How to contribute

1. Fork the repo and create a branch
2. Add Javadoc to the four methods above
3. Run ./mvnw spotless:apply and ./mvnw -q -DskipTests verify
4. Open a PR with title: [Docs][Core] Add Javadoc to MultiTableWriterRunnable