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>

tests/unit/vector/test_errors.py

@@ -22,6 +22,9 @@ def test_format_exception_group_names_leaf_cause():
     formatted = format_exception_group(group)
 
     assert "ConnectError" in formatted
+    # Assert the full leaf repr survives, not just the type name -- guards a
+    # future format change that kept the type but dropped the message.
+    assert repr(leaf) in formatted
     assert "unhandled errors in a TaskGroup" not in formatted
     assert "1 sub-exception" in formatted