refactor: drop StreamingResultSet getObject(Class) bandaid

Now that QueryJDBCAccessor.getObject(Class) provides the raw + isInstance
fallback as its base-class default, StreamingResultSet no longer needs
the catch-and-retry path that worked around accessors which threw
"Operation not supported." Collapse getObject(int, Class) to direct
dispatch and update the regression test's WHY comment to point at the
accessor base class as the load-bearing layer.

Addresses: review comment on PR #175 line 388.

Author: mkaufmann

jdbc-core/src/main/java/com/salesforce/datacloud/jdbc/core/StreamingResultSet.java

@@ -360,32 +360,10 @@ public Object getObject(int columnIndex, Map<String, Class<?>> map) throws SQLEx
 
     @Override
     public <T> T getObject(int columnIndex, Class<T> type) throws SQLException {
-        if (type == null) {
-            throw new SQLException("Target type must not be null");
-        }
-        // Default implementation: get the raw Object and check if it matches the requested type.
-        // Accessors that need richer conversion (e.g. Arrow Decimal → BigInteger) can still
-        // override the accessor-level getObject(Class) — in that case we dispatch to them first.
         val accessor = getAccessor(columnIndex);
-        try {
-            val typed = accessor.getObject(type);
-            updateWasNull(accessor);
-            return typed;
-        } catch (SQLException ex) {
-            // Accessor does not implement typed getObject — fall back to raw + isInstance check.
-            val raw = accessor.getObject();
-            updateWasNull(accessor);
-            if (raw == null) {
-                return null;
-            }
-            if (type.isInstance(raw)) {
-                return type.cast(raw);
-            }
-            throw new SQLException(
-                    "Cannot convert column value to " + type.getName() + "; actual type is "
-                            + raw.getClass().getName(),
-                    ex);
-        }
+        val result = accessor.getObject(type);
+        updateWasNull(accessor);
+        return result;
     }
 
     @Override