fix(vector): address PR review round 9 — drop redundant guard, add init lock, test float doc_id path

Addresses the four 🟡 important findings from claude-bot review on PR #773:

str (non-Optional) and the guard would silently skip the Qdrant lookup
for an empty string. Removing the guard matches the type signature.

(`all([…, doc_id, …])` rejects None and empty string, plus
`assert doc_id is not None`). No code change needed.

`get_qdrant_client()` with a module-level `anyio.Lock`. Double-checked
locking keeps the steady-state hot path lock-free. Without this,
parallel cold-start callers could all enter the init block and run
`_backfill_doc_id_to_string` + `_ensure_payload_indexes` redundantly
(idempotent, but noisy). Pattern matches `auth/storage.py:2071`.

behavior with three tests covering the float-warning path (the gap
called out in the review), the str/None silent-skip paths, and the
int-grouping happy path.

Verification:
- ruff check / format: clean
- ty check -- nextcloud_mcp_server: clean
- uv run pytest tests/unit/: 969 passed

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

.claude/skills/pre-push-review/SKILL.md

@@ -7,7 +7,6 @@ description: |
   in this repo's automated PR reviews. Use when the user is about to push, says "ready
   to push", "review my work", "check before PR", or invokes /pre-push-review.
   Report-only — does not modify code.
-model: sonnet
 allowed-tools:
   - Bash
   - Read