fix: _poll_oneshot for DirSize/MD5 with burst-retry on early 599

Add FileStationClient.start_and_poll_immediately: starts the async task and
immediately makes the first status poll within the same method, with no
intermediate awaits other than the two HTTP calls. This minimises scheduler
latency between start and first poll for one-shot tasks.

_poll_oneshot now accepts the first_status from start_and_poll_immediately:
- finished=True on first poll → return immediately
- finished=False → Phase 2 (exponential backoff, 60 s timeout)
- None (first poll was 599) → burst-retry 10× at 10 ms, then Phase 2
  (Phase 2 keeps polling through 599 until seen_alive, then fails fast)

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__ = "0.2.6"
+__version__ = "0.2.7"