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/providers/test_retry.py

@@ -39,8 +39,8 @@ async def test_retry_succeeds_after_429():
 
 
 @pytest.mark.unit
-async def test_retry_reraises_non_rate_limit_immediately():
-    """A non-rate-limit error of the same class is re-raised on first hit."""
+async def test_retry_reraises_when_predicate_returns_false():
+    """An error the predicate rejects is re-raised on first hit (no retry)."""
     calls = {"n": 0}
 
     @_retry.retry_on_transient(_FakeError, should_retry=lambda e: e.status_code == 429)