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>

src/mcp_synology_filestation/__init__.py

@@ -1,3 +1,3 @@
 """MCP server for Synology FileStation."""
 
-__version__ = "0.2.8"
+__version__ = "0.2.9"