fix(vector): guard unbound doc_task + address review nits (#891)

Round-1 review on PR #891:
- Guard processor_task's broad except handler against an unbound doc_task
  (mirrors multi_user_processor_task): initialise doc_task=None before the loop
  and branch the error log. Fixes a latent NameError if receive() raises a
  non-TimeoutError/EndOfStream before the first document binds. Regression test
  added.
- Drop the unnecessary `from __future__ import annotations` in vector/_errors.py
  and express format_exception_group's non-group fast path as an explicit
  isinstance check.
- Add a copy_resource Destination-header encoding test (analogue to MOVE);
  strengthen the ExceptionGroup test to assert the full leaf repr survives.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/vector/_errors.py

@@ -9,8 +9,6 @@ exceptions so log lines name the actual cause; pair it with ``exc_info=True`` to
 keep the full traceback.
 """
 
-from __future__ import annotations
-
 
 def format_exception_group(exc: BaseException) -> str:
     """Return a concise, leaf-naming string for ``exc``.
@@ -20,9 +18,9 @@ def format_exception_group(exc: BaseException) -> str:
     result is meant for the human-readable portion of a log message, not for
     parsing.
     """
-    leaves = _flatten(exc)
-    if len(leaves) == 1 and leaves[0] is exc:
+    if not isinstance(exc, BaseExceptionGroup):
         return repr(exc)
+    leaves = _flatten(exc)
     return f"{len(leaves)} sub-exception(s): " + "; ".join(repr(e) for e in leaves)