fix(search): address PR #834 review findings

Resolve the latest PR-review comment on the verify-on-read tag-gate work:

- tests: stringify note IDs in test_verify_on_read.py so SearchResult.id
  matches production (scanner stringifies all IDs on write) — helper and the
  keeps/deleted/mixed/dedupe assertions (blocking).
- tests: make the unshared-file negative control a PDF so the drop is
  unambiguously "unshared", not a mime_type_filter mismatch.
- config: add Validator("VECTOR_SYNC_PDF_TAG", len_min=1) — an empty tag name
  would make find_files_by_tag("") misbehave in the verifier and scanner.
- verification: correct the _verify_files comment — two batch fetches (tag
  REPORT + EXCLUDED_TAGS lookup) are held under one semaphore slot; the
  pure-Python intersection runs outside it.
- tests: de-duplicate the minimal-PDF constant into a shared PDF_BYTES in
  tests/integration/conftest.py, imported by both integration modules.

Verified: ruff/format/ty/unit all green; the two integration modules
(10 tests) pass against a local Nextcloud (app-only, no MCP profile needed).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/integration/conftest.py

@@ -13,6 +13,19 @@ logger = logging.getLogger(__name__)
 # Valid provider names
 VALID_PROVIDERS = ["openai", "ollama", "anthropic", "bedrock"]
 
+# Canonical minimal valid PDF for integration tests. verify-on-read gates file
+# results on the vector-index tag via
+# find_files_by_tag(..., mime_type_filter="application/pdf"), so file fixtures
+# must be PDFs (matching what the scanner indexes), not .txt. Shared here so the
+# constant is defined once rather than drifting across test modules.
+PDF_BYTES = (
+    b"%PDF-1.4\n"
+    b"1 0 obj<</Type/Catalog/Pages 2 0 R>>endobj\n"
+    b"2 0 obj<</Type/Pages/Kids[3 0 R]/Count 1>>endobj\n"
+    b"3 0 obj<</Type/Page/Parent 2 0 R/MediaBox[0 0 200 200]>>endobj\n"
+    b"trailer<</Root 1 0 R>>\n%%EOF\n"
+)
+
 
 def pytest_addoption(parser):
     """Add --provider command line option for RAG tests."""