fix: retry _poll_task on transient 599 instead of aborting immediately

DirSize for large directories (e.g. /docker, 8441 folders, 46832 files)
takes ~800ms to compute. While running, status returns intermediate
progress (finished=false). But on the very first poll the task can return
599 transiently (task just started, not yet available). Previously
_poll_task caught any SynologyError and returned immediately, making
dir_size always fail on the first 599.

Fix: treat 599 as a transient condition and continue polling. Give up
only after 5 consecutive 599 responses. All other error codes remain
immediately fatal.

Investigation confirmed with test_dirsize_md5.py:
- /test-mcp (2937 B): finished=true at 0ms
- /docker (3.9 GB, 46832 files): finished=false at 35ms, finished=true at 789ms

Tests: 2 new cases (retry-succeeds, 5x-599-gives-up) → 95 total

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

src/mcp_synology_filestation/tools/filestation.py

@@ -86,6 +86,7 @@ def register_filestation(
         delay = 0.2
         elapsed = initial_delay
         timeout = 60.0
+        consecutive_599 = 0
 
         if initial_delay > 0:
             await asyncio.sleep(initial_delay)
@@ -98,11 +99,19 @@ def register_filestation(
                     version=version,
                     params={"taskid": taskid},
                 )
+                consecutive_599 = 0
             except _SynologyError as e:
-                return False, f"Error: {e}"
-
-            if status_data.get("finished"):
-                return True, status_data
+                if e.code == 599:
+                    # 599 can be transient (task just started, not yet available).
+                    # Retry up to 5 times before giving up.
+                    consecutive_599 += 1
+                    if consecutive_599 >= 5:
+                        return False, f"Error: {e}"
+                else:
+                    return False, f"Error: {e}"
+            else:
+                if status_data.get("finished"):
+                    return True, status_data
 
             if elapsed >= timeout:
                 return (