fix(search): cap path_prefixes server-side; unify Iterable typing

Round 3 review follow-ups:
- Enforce the folder cap (MAX_PATH_PREFIXES=20) inside normalize_path_prefixes
  so the REST/viz endpoints are bounded too, not just the MCP tool's Field
  and the PHP client. Single server-side enforcement point; the MCP tool's
  Field(max_length=...) now references the same constant.
- Widen the SearchAlgorithm ABC and both concrete implementations'
  path_prefixes param to Iterable[str] | None, matching the widening of
  build_base_filter_conditions from the prior round.
- Add a normalize_path_prefixes cap test.

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

nextcloud_mcp_server/server/semantic.py

@@ -33,6 +33,7 @@ from nextcloud_mcp_server.observability.metrics import (
     instrument_tool,
 )
 from nextcloud_mcp_server.search.access_filter import (
+    MAX_PATH_PREFIXES,
     list_accessible_owners,
     normalize_path_prefixes,
 )
@@ -102,14 +103,14 @@ def configure_semantic_tools(mcp: FastMCP):
         path_prefixes: Annotated[
             list[str] | None,
             Field(
-                max_length=20,
+                max_length=MAX_PATH_PREFIXES,
                 description=(
                     "Restrict to files under any of these folders/paths "
                     "(e.g. ['/Projects/Reports', '/Shared/Specs']). Folders are "
                     "OR-ed together. Matches the file_path of indexed files "
                     "only, so setting it implicitly limits results to files. "
-                    "Capped at 20 folders to bound the OR-filter width. "
-                    "None or empty = no path filter."
+                    f"Capped at {MAX_PATH_PREFIXES} folders to bound the "
+                    "OR-filter width. None or empty = no path filter."
                 ),
             ),
         ] = None,