security(mcp): find_documents and count_documents can read system.* collections that list_collections hides #1774

Problem

`McpMongoService.listCollections` drops names starting with `system.`, but `_knownCollection`, which validates the collection of `find_documents` and `count_documents`, checks against the unfiltered `session.listCollections()`. A client can name `system.profile` (past queries with their literals), `system.views` or any `system.*` collection of the connection's database even though it is hidden from the list.

Fix

Apply the same `system.` filter in `_knownCollection` (one helper used by both). Test: `find` and `count` on `system.profile` are refused with the same message as an unknown collection.

Found in the MCP review (see #1769 for the SQL guard).