refactor(search): address PR #750 round 7 review feedback

Round 7 raised 5 issues; this round addresses all of them and fixes
the underlying causes (not just the comments) where applicable so
they don't get re-flagged in future passes.

Critical:
- verified_count description in SemanticSearchResponse said "unique
  documents" but the value is len(verified_results), a chunk count.
  Description rewritten to accurately document chunk-level granularity
  AND explicitly call out the asymmetry with dropped_count (which
  counts unique (doc_id, doc_type) pairs).

- _verify_files false-eviction risk: the round-6 doc-only fix was
  re-flagged. Address at the source — widen WebDAVClient.get_file_info
  to raise HTTPStatusError on 404 (matching the rest of the client
  convention) and reserve None for the genuinely ambiguous
  malformed-PROPFIND case. _verify_files now keeps the result on None
  (cannot tell whether the file exists) and evicts only on a
  definitive HTTPStatusError 404. Tests updated; new test added for
  the malformed-XML keep-result path.

Non-critical:
- News verifier semaphore lifetime now explicitly documented: one
  slot held for one deduplicated fetch per search is the correct
  backpressure behaviour.

- Cross-reference comments in _verify_notes / _verify_deck_cards no
  longer claim "Mirrors X" pointing at functions defined later in
  the file; now use direction-neutral "parallel implementation in".

- accessible_by_type is mutated by concurrent run_verifier tasks; a
  comment explains why this is race-free under anyio's cooperative
  multitasking (distinct keys per task, no await between read and
  write) so a future reader doesn't add a redundant lock.

- Knock-on: tests/integration/test_rag.py wraps get_file_info in a
  try/except for the new contract.

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

nextcloud_mcp_server/client/webdav.py

@@ -1306,7 +1306,19 @@ class WebDAVClient(BaseNextcloudClient):
 
         Returns:
             File info dictionary with id, name, size, content_type, etc.
-            Returns None if file not found.
+            Returns ``None`` ONLY when the server returned a malformed
+            PROPFIND response (missing ``<d:response>`` /
+            ``<d:propstat>`` / ``<d:prop>`` elements) — an ambiguous
+            state where we cannot tell whether the file exists.
+
+        Raises:
+            HTTPStatusError: For any non-2xx HTTP status, including 404
+                ("not found"). Callers that want to treat 404 as
+                "absent" should catch ``HTTPStatusError`` and check
+                ``e.response.status_code``. This matches the convention
+                of the rest of this client and lets verify-on-read
+                distinguish a definitive absence (HTTP 404) from a
+                brittle response (None).
         """
         webdav_path = f"{self._get_webdav_base_path()}/{path.lstrip('/')}"
 
@@ -1323,19 +1335,13 @@ class WebDAVClient(BaseNextcloudClient):
   </d:prop>
 </d:propfind>"""
 
-        try:
-            response = await self._client.request(
-                "PROPFIND",
-                webdav_path,
-                headers={"Depth": "0"},
-                content=propfind_body,
-            )
-            response.raise_for_status()
-        except HTTPStatusError as e:
-            if e.response.status_code == 404:
-                logger.debug(f"File not found: {path}")
-                return None
-            raise
+        response = await self._client.request(
+            "PROPFIND",
+            webdav_path,
+            headers={"Depth": "0"},
+            content=propfind_body,
+        )
+        response.raise_for_status()
 
         # Parse XML response
         root = ET.fromstring(response.content)