fix: use correct status API versions for DirSize and MD5

- DirSize: _poll_task now uses status version=1 (v2 always returns 599)
- MD5: _poll_task keeps status version=2 (confirmed working via live NAS test)

Investigation notes documented in test_dirsize_md5.py:
both APIs use start v2; DirSize status needs v1, MD5 status needs v2;
tiny data causes one-shot race condition (no issue with real-world data).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/mcp_synology_filestation/tools/filestation.py

@@ -805,7 +805,7 @@ def register_filestation(
         if not taskid:
             return "Error: DSM did not return a task ID."
 
-        ok, result = await _poll_task("SYNO.FileStation.DirSize", 2, taskid)
+        ok, result = await _poll_task("SYNO.FileStation.DirSize", 1, taskid)
         if not ok:
             return result  # type: ignore[return-value]