[Java][FlightRPC] Flight SQL JDBC driver does not expose per-batch app_metadata from FlightStream

### Summary
The Flight SQL JDBC driver (flight-sql-jdbc-driver) currently exposes FlightInfo.app_metadata via ArrowFlightJdbcFlightStreamResultSet.getAppMetadata() (added in ~15.0), but per-batch app_metadata from FlightStream.getLatestMetadata() is received internally and silently discarded — it is never surfaced to JDBC callers.

### Motivation
Servers use per-batch app_metadata on FlightData messages to communicate execution statistics (rows scanned, elapsed time, query state, etc.) — similar to what Trino exposes via QueryStats. This is particularly valuable for the final batch, which servers commonly use as a "trailing metadata" pattern to report post-execution stats.
Currently there is no way to access this information through the JDBC layer without dropping down to the raw FlightClient, which defeats the purpose of the JDBC driver abstraction.

### Current behavior

FlightInfo.app_metadata → accessible via rs.unwrap(ArrowFlightJdbcFlightStreamResultSet.class).getAppMetadata() ✅
Per-batch FlightData.app_metadata → received by FlightStream.getLatestMetadata() internally, but never stored or exposed ❌

**Also worth noting**: getAppMetadata() returns a snapshot from query submission time only. For long-running queries using PollFlightInfo, updated app_metadata from subsequent poll cycles is never surfaced. This limits its usefulness for engines that progressively update stats during execution.

### Requested change
Add a method to ArrowFlightJdbcFlightStreamResultSet (accessible via unwrap()) such as:
```
public ArrowBuf getLatestBatchMetadata() {
if (currentEndpointData == null || currentEndpointData.getStream() == null) {
return null;
}
return currentEndpointData.getStream().getLatestMetadata();
}
```
This would follow the same unwrap() pattern already established for getAppMetadata() and give callers access to per-batch metadata, including trailing stats on the final batch.

### Component(s)

FlightRPC, Java

Beginne mit ArrowFlightJdbcFlightStreamResultSet und seiner vorhandenen getAppMetadata()-Methode und verfolge dann currentEndpointData und FlightStream.getLatestMetadata(). Die Aufgabe ist abgeschlossen, wenn JDBC-Aufrufer unwrap() verwenden können, um die neuesten Metadaten pro Batch abzurufen, einschließlich der abschließenden Metadaten des letzten Batches, ohne auf den rohen FlightClient zuzugreifen.

Vom Indexierungsmodell aus dem Issue-Text verfasst.