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>

tests/test_tools_filestation.py

@@ -1610,6 +1610,49 @@ async def test_dir_size_empty_path(config: AppConfig) -> None:
     client.request.assert_not_called()
 
 
+@pytest.mark.asyncio
+async def test_dir_size_retries_on_transient_599(config: AppConfig) -> None:
+    """dir_size retries up to 4 times on code-599 then succeeds on 5th status call."""
+    client = MagicMock()
+    call_count = {"status": 0}
+
+    async def _request(api, method, version=None, params=None, **kwargs):
+        if method == "start":
+            return {"taskid": "FileStation_dirsize_599"}
+        call_count["status"] += 1
+        if call_count["status"] < 4:
+            raise SynologyError("DSM error code 599", code=599)
+        return {"finished": True, "num_dir": 2, "num_file": 10, "total_size": 1024}
+
+    client.request = AsyncMock(side_effect=_request)
+    tools = _make_mcp_and_tools(config, client)
+
+    with patch("asyncio.sleep", new_callable=AsyncMock):
+        result = await tools["dir_size"](path="/data")
+
+    assert "Total Size" in result
+    assert call_count["status"] == 4
+
+
+@pytest.mark.asyncio
+async def test_dir_size_fails_after_5_consecutive_599(config: AppConfig) -> None:
+    """dir_size gives up and returns Error: after 5 consecutive 599 responses."""
+    client = MagicMock()
+
+    async def _request(api, method, version=None, params=None, **kwargs):
+        if method == "start":
+            return {"taskid": "FileStation_dirsize_dead"}
+        raise SynologyError("DSM error code 599", code=599)
+
+    client.request = AsyncMock(side_effect=_request)
+    tools = _make_mcp_and_tools(config, client)
+
+    with patch("asyncio.sleep", new_callable=AsyncMock):
+        result = await tools["dir_size"](path="/dead")
+
+    assert result.startswith("Error:")
+
+
 # ──────────────────────────────────────────────────────────────────────────
 # get_md5
 # ──────────────────────────────────────────────────────────────────────────