Add ConnzConnectionInfo, ConnzFilterResult, ConnzFilterOptions, and
ConnzFilter static class to Connz.cs, providing a pure unit-testable
layer for account-scoped filtering and pagination that mirrors the Go
server's /connz ?acc= query-parameter behaviour. Ten new tests in
ConnzAccountFilterTests.cs cover FilterByAccount (match, no-match,
case-insensitive), ConnzFilterOptions.Parse (acc param, defaults,
offset/limit), and ApplyFilters (account filter, offset, limit,
no-filter pass-through).
