fix(vector): address PR review round 8 — anyio convention + cosine-safe sentinel + dedup get_collection

Reviewer findings (1 blocking + 2 important):

- 🔴 Replace `import asyncio` / `await asyncio.sleep(0)` with
  `import anyio` / `await anyio.sleep(0)` in the four async side-effect
  helpers (_scroll_raises, _upsert_raises, _get_collection_raises,
  _create_index). CLAUDE.md mandates anyio for all async operations;
  conftest pins the backend to asyncio so the asyncio.sleep call worked
  today, but the inconsistency would surface the moment that pin moves.
- 🟡 Replace the sentinel's zero dense vector with a single non-zero
  element (`[1e-9] + [0.0] * (dimension - 1)`). Cosine distance is
  mathematically undefined for the zero vector and Qdrant Cloud strict
  mode rejects zero-vector upserts. The exact value doesn't matter
  (sentinel never participates in a search — no user_id/doc_id/doc_type
  payload) but the upsert itself must be valid.
- 🟡 Avoid the duplicate `get_collection` round-trip on every restart.
  `_ensure_payload_indexes` now accepts an optional
  `existing_schema: dict | None` parameter; when None it fetches
  collection_info itself (and the get_collection-failure swallow still
  applies), but `get_qdrant_client` already fetches collection_info
  for dimension validation in the existing-collection branch — pass
  `collection_info.payload_schema or {}` through to skip the second
  call. The new-collection branch passes `existing_schema={}`
  explicitly since a freshly created collection has no payload schema.

The 🟡 deck_card iteration-fallback finding doesn't apply: the
`isdigit()` guard at context.py:612 returns early before either the
fast-path or the iteration fallback runs, so non-numeric doc_ids
cannot reach the inner `c.id == int(doc_id)` comparison.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/unit/vector/test_qdrant_client.py

@@ -14,10 +14,10 @@ run at startup. Producer-side normalization is exercised by the existing
 scanner tests.
 """
 
-import asyncio
 from types import SimpleNamespace
 from unittest.mock import call
 
+import anyio
 import httpx
 import pytest
 from qdrant_client.http.exceptions import UnexpectedResponse
@@ -237,7 +237,7 @@ async def test_ensure_payload_indexes_logs_and_returns_when_get_collection_raise
 
     async def _get_collection_raises(*args, **kwargs):
         # See _scroll_raises in the backfill section for why this is async.
-        await asyncio.sleep(0)
+        await anyio.sleep(0)
         raise RuntimeError("connection refused")
 
     client.get_collection.side_effect = _get_collection_raises
@@ -481,11 +481,11 @@ async def test_backfill_logs_and_returns_when_scroll_raises(mocker, caplog):
     # An async-callable side_effect lets AsyncMock await the coroutine
     # before the exception propagates; assigning a bare exception class
     # leaks an un-awaited coroutine and trips RuntimeWarning at gc time.
-    # The `await asyncio.sleep(0)` is a no-op event-loop yield that
+    # The `await anyio.sleep(0)` is a no-op event-loop yield that
     # satisfies static analysis ("async function uses no async features")
     # without changing observable behavior.
     async def _scroll_raises(*args, **kwargs):
-        await asyncio.sleep(0)
+        await anyio.sleep(0)
         raise RuntimeError("boom")
 
     client.scroll.side_effect = _scroll_raises
@@ -525,7 +525,7 @@ async def test_backfill_logs_warning_when_sentinel_upsert_fails(mocker, caplog):
 
     async def _upsert_raises(*args, **kwargs):
         # See _scroll_raises above for why this is async + sleep(0).
-        await asyncio.sleep(0)
+        await anyio.sleep(0)
         raise RuntimeError("sentinel write blip")
 
     client.upsert.side_effect = _upsert_raises
@@ -602,7 +602,7 @@ async def test_ensure_payload_indexes_summarises_failed_fields(mocker, caplog):
 
     async def _create_index(*args, **kwargs):
         # See _scroll_raises above for why this is async + sleep(0).
-        await asyncio.sleep(0)
+        await anyio.sleep(0)
         call_count["n"] += 1
         if call_count["n"] != 2:
             raise _make_unexpected(500, b'{"status":{"error":"boom"}}')