Fix mapping leak on openExisting error path

openExisting creates the RW mapping only after the recovery scan
completes. The catch block used to close the fd but not the mapping,
so a throw between a successful Files.mmap and the constructor return
(realistically only an OOME allocating the MmapSegment) leaked the
mapping. Track the address across the try block and munmap it in the
catch before closing the fd, mirroring the cleanup pattern create()
uses.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: mtopolnik

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

@@ -296,6 +296,7 @@ public static MmapSegment openExisting(FilesFacade ff, String path) {
         if (fd < 0) {
             throw new MmapSegmentException("openRW failed for " + path);
         }
+        long addr = Files.FAILED_MMAP_ADDRESS;
         try {
             RecoveryScan scan = scanFile(ff, fd, path, fileSize);
             if (scan.tornTailBytes > 0) {
@@ -306,13 +307,16 @@ public static MmapSegment openExisting(FilesFacade ff, String path) {
                                 + "Investigate disk health or unexpected writer crash.",
                         path, scan.tornTailBytes, scan.lastGood, fileSize, scan.frameCount);
             }
-            long addr = Files.mmap(fd, fileSize, 0, Files.MAP_RW, MemoryTag.MMAP_DEFAULT);
+            addr = Files.mmap(fd, fileSize, 0, Files.MAP_RW, MemoryTag.MMAP_DEFAULT);
             if (addr == Files.FAILED_MMAP_ADDRESS) {
                 throw new MmapSegmentException("mmap failed for " + path);
             }
             return new MmapSegment(path, fd, addr, fileSize, scan.baseSeq,
                     scan.lastGood, scan.frameCount, false, scan.tornTailBytes);
         } catch (Throwable t) {
+            if (addr != Files.FAILED_MMAP_ADDRESS) {
+                Files.munmap(addr, fileSize, MemoryTag.MMAP_DEFAULT);
+            }
             ff.close(fd);
             throw t;
         }