fix(search): cap path_prefixes at the MCP tool; widen path filter tests

Round 2 review follow-ups:
- Add Field(max_length=20) to the nc_semantic_search path_prefixes param so
  an LLM client can't build an unbounded OR-filter (mirrors the cap the
  Astrolabe PHP controller applies on the UI path).
- Note in normalize_path_prefixes that the two-pass collect-then-strip is
  deliberate (the `if path_prefix:` guard is truthy for whitespace-only
  input; the strip pass is what drops it).
- Tests: exercise build_base_filter_conditions with 3 folders (guards the
  list comprehension) and parametrize the no-path case over None, empty
  list, and blank-only inputs.

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

nextcloud_mcp_server/server/semantic.py

@@ -102,11 +102,13 @@ def configure_semantic_tools(mcp: FastMCP):
         path_prefixes: Annotated[
             list[str] | None,
             Field(
+                max_length=20,
                 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."
                 ),
             ),