fix: cold-start 599 on DirSize/MD5 — restart task instead of giving up

DSM's DirSize and MD5 background service needs ~6-8 s to initialise
after a period of inactivity. During this cold-start window tasks are
registered but every status poll returns error 599 ("no such task").

Replace the ad-hoc start+_poll_task call in dir_size and get_md5 with
a new _start_and_poll_oneshot helper that:
- polls with exponential backoff (0.2 s → cap 2 s)
- on 5 consecutive 599s: restarts the task (up to 6 attempts total)
- honours a shared 60 s wall-clock budget across all restarts
- returns a clear error if all restart attempts are exhausted

Root cause confirmed by test_dirsize_md5.py: after ~6 s / 2 restarts
the service warms up and the very first poll on the new task succeeds.

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

tests/test_tools_filestation.py

@@ -1636,7 +1636,7 @@ async def test_dir_size_retries_on_transient_599(config: AppConfig) -> None:
 
 @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."""
+    """dir_size gives up and returns Error: after exhausting all restart attempts."""
     client = MagicMock()
 
     async def _request(api, method, version=None, params=None, **kwargs):
@@ -1653,6 +1653,34 @@ async def test_dir_size_fails_after_5_consecutive_599(config: AppConfig) -> None
     assert result.startswith("Error:")
 
 
+@pytest.mark.asyncio
+async def test_dir_size_cold_start_restart(config: AppConfig) -> None:
+    """dir_size restarts the task after 5 consecutive 599s and succeeds on second attempt."""
+    client = MagicMock()
+    start_count = {"n": 0}
+    status_count = {"n": 0}
+
+    async def _request(api, method, version=None, params=None, **kwargs):
+        if method == "start":
+            start_count["n"] += 1
+            return {"taskid": f"task_{start_count['n']}"}
+        status_count["n"] += 1
+        # First 5 status calls → 599 (simulates cold start)
+        if status_count["n"] <= 5:
+            raise SynologyError("DSM error code 599", code=599)
+        # After restart: immediately done
+        return {"finished": True, "num_dir": 1, "num_file": 5, "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="/coldstart")
+
+    assert "Total Size" in result
+    assert start_count["n"] == 2  # task was restarted once after cold-start 599s
+
+
 # ──────────────────────────────────────────────────────────────────────────
 # get_md5
 # ──────────────────────────────────────────────────────────────────────────