fix(search): address review feedback on multi-folder path filter

- visualization.py: drop the CSV string-split branch. The Astrolabe PHP
  client sends path_prefixes as a JSON array, so only a list is accepted;
  any other shape is ignored rather than comma-split (which would corrupt
  folder names containing commas).
- viz_routes.py: split the path_prefixes query param on newline (a comma
  is a valid POSIX path char; a newline is not) and pass None instead of
  [""] when the param is absent.
- access_filter.py: widen build_base_filter_conditions' path_prefixes to
  Iterable[str] for consistency with normalize_path_prefixes.
- ADR-027: document the newline delimiter (frontend/viz route) and JSON
  array (PHP->MCP body), and the PHP-side cap on list width.

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

nextcloud_mcp_server/search/access_filter.py

@@ -234,7 +234,7 @@ def build_base_filter_conditions(
     modified_after: int | None = None,
     modified_before: int | None = None,
     path_prefix: str | None = None,
-    path_prefixes: list[str] | None = None,
+    path_prefixes: Iterable[str] | None = None,
 ) -> list[Condition]:
     """Build the common ``must`` conditions shared by every search algorithm.