test(vector): close generate()/drop-counter test gaps + https mock URLs (#893)

Round-2 review on PR #893:
- Add test_generate_retries_on_connection_error (generate() shares the transient
  retry; guards the decorator against accidental removal).
- Add test_process_document_records_drop_on_exhausted_retries: drives
  process_document to retry-exhaustion and asserts record_ingest_dropped is
  called once with the classified reason (processor-level coverage, not just the
  _drop_reason unit).
- Note in _drop_reason that a multi-failure group is labelled by its first leaf
  (best-effort, no "mixed" bucket).

SonarCloud: the quality gate was failing on new_security_hotspots_reviewed
(S5332 "use https") from http:// URLs in the test _req() helpers — switched to
https:// (mirrors commit 98c9d58e).

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

nextcloud_mcp_server/vector/processor.py

@@ -69,7 +69,9 @@ def _drop_reason(exc: BaseException) -> str:
     unknown causes fall back to ``other``.
     """
     # An anyio task group can wrap the real cause (and nest groups when sub-tasks
-    # use their own groups); descend to the first concrete leaf.
+    # use their own groups); descend to the first concrete leaf. Best-effort: a
+    # group bundling several distinct failures is labelled by whichever leaf
+    # sorts first, not by a "mixed" bucket.
     while isinstance(exc, BaseExceptionGroup) and exc.exceptions:
         exc = exc.exceptions[0]