fix(vector): nested-group drop classification + review/Sonar fixes (#893)

Round-1 review on PR #893:
- _drop_reason now descends through nested ExceptionGroups to the first leaf
  (was single-level), so a doubly-wrapped cause isn't mislabelled "other";
  added a nested-group test. Commented why both the httpx and openai isinstance
  branches exist (raw Nextcloud-API errors vs SDK-wrapped variants).
- Documented that generate() intentionally shares the broadened transient retry
  (RAG sampling path), with the worst-case latency note.
- Added a docstring note to process_document on how the provider-level retry
  (5x) layers over the outer loop (3x in-process / 1x procrastinate).
- Added test_embed_batch_retries_on_connection_error for the batch path.
- Renamed test_retry_reraises_non_rate_limit_immediately ->
  test_retry_reraises_when_predicate_returns_false (it tests the predicate, not
  a specific status).

SonarCloud:
- S5708 (BLOCKER) on the helper's dynamic `except exception_type`: the type is
  constrained to BaseException/tuple by the signature; suppressed with a
  justified NOSONAR.
- S7503 (async without await) in the embed-retry test: use AsyncMock side_effect
  instead of a hand-rolled async function.

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

tests/unit/test_processor_drop_reason.py

@@ -59,6 +59,15 @@ def test_exception_group_unwraps_to_leaf():
     assert processor._drop_reason(group) == "connection"
 
 
+@pytest.mark.unit
+def test_nested_exception_group_descends_to_leaf():
+    """A doubly-wrapped group must still classify by its leaf, not 'other'."""
+    nested = BaseExceptionGroup(
+        "outer", [BaseExceptionGroup("inner", [httpx.ReadTimeout("slow")])]
+    )
+    assert processor._drop_reason(nested) == "timeout"
+
+
 @pytest.mark.unit
 def test_qdrant_namespace_classified():
     from qdrant_client.http.exceptions import UnexpectedResponse