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>

tests/integration/test_rag.py

@@ -38,6 +38,7 @@ from typing import Any, AsyncGenerator
 
 import anyio
 import pytest
+from httpx import HTTPStatusError
 from mcp import ClientSession
 
 from nextcloud_mcp_server.providers.base import Provider
@@ -130,10 +131,18 @@ async def indexed_manual_pdf(nc_client, nc_mcp_client):
 
     logger.info(f"Setting up indexed manual PDF: {manual_path}")
 
-    # Get file info to verify file exists and get file ID
-    file_info = await nc_client.webdav.get_file_info(manual_path)
+    # Get file info to verify file exists and get file ID. After the
+    # round-7 contract widening, get_file_info raises HTTPStatusError on
+    # 404 instead of returning None — so wrap and skip on a definitive
+    # not-found.
+    try:
+        file_info = await nc_client.webdav.get_file_info(manual_path)
+    except HTTPStatusError as e:
+        if e.response.status_code == 404:
+            pytest.skip(f"Manual PDF not found at '{manual_path}'")
+        raise
     if not file_info:
-        pytest.skip(f"Manual PDF not found at '{manual_path}'")
+        pytest.skip(f"Manual PDF unreadable at '{manual_path}' (malformed PROPFIND)")
 
     file_id = file_info["id"]
     logger.info(f"Found manual PDF: {manual_path} (file_id={file_id})")