Add entity count to list_entities response

[mgoldsborough]

Summary

When using the MCP server, list_{plural} returns an array of entities but doesn't tell you the total count. Adding a wrapper response with count and entities fields would help agents understand the dataset size.

What to do

1. In lib/src/nb_pathway/server.py, update the list_tool function
2. Change the return type from list[dict] to dict with structure:

   {
     "count": 5,
     "entities": [...]
   }

3. Do the same for search_tool
4. Update tests in lib/tests/test_server.py

Acceptance criteria

• list_{plural} returns {"count": N, "entities": [...]}
• search_{plural} returns {"count": N, "entities": [...]}
• Tests updated
• make check passes
• E2E tests still pass

Estimated time: ~1-2 hours