fix: resolve all type checking errors (8 errors fixed)

Fixed 8 type checker errors across the codebase:

- vector/scanner.py: Handle None scroll results with null-safe iteration
- search/{bm25_hybrid,semantic}.py: Add None checks for result.payload
- auth/{unified_verifier,webhook_routes}.py: Assert non-None auth credentials
- client/webdav.py: Add None checks before int() conversions
- providers/openai.py: Assert embedding_model is not None
- search/algorithms.py: Explicitly type doc_types set and cast values
- observability/logging_config.py: Match parent class signature (log_data)

Also fixed test_create_tag_creates_system_tag to match WebDAV implementation
(was testing OCS API endpoint, now tests correct WebDAV endpoint with
Content-Location header).

Type checker: 0 errors (down from 8), 20 warnings (ignored)
Tests: All 192 unit tests passing

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

nextcloud_mcp_server/auth/webhook_routes.py

@@ -139,6 +139,7 @@ async def _get_authenticated_client(request: Request) -> httpx.AsyncClient:
             raise RuntimeError("BasicAuth credentials not configured")
 
         assert nextcloud_host is not None  # Type narrowing for type checker
+        assert username is not None and password is not None  # Type narrowing
         return httpx.AsyncClient(
             base_url=nextcloud_host,
             auth=(username, password),