Add comprehensive unit tests for lib package (90.3% coverage)

[Copilot]

The lib package had no test coverage. This PR adds comprehensive unit tests achieving 90.3% coverage using only Go's standard testing package.

Test Files Added

• error_test.go - Error constant validation
• lib_test.go - Constants, types, and registry tests
• common_test.go - HTTP utilities and JSON unmarshaling (with httptest mock servers)
• config_test.go - Config creator registration and unmarshaling
• converter_test.go - Converter registration
• entry_test.go - IP prefix processing, marshaling, and set operations
• container_test.go - Container operations, merging, and lookup
• instance_test.go - Instance lifecycle and config loading

Coverage Breakdown

Key functions tested:

• Entry.processPrefix() - All IP types: net.IP, *net.IPNet, netip.Addr, netip.Prefix, strings with CIDR notation
• Container.Add() - Entry merging with IgnoreIPv4/IgnoreIPv6 options
• Container.Remove() - Both CaseRemovePrefix and CaseRemoveEntry modes
• Container.Lookup() - IPv4/IPv6 address and CIDR lookups with search lists
• Instance.InitConfig() - File and HTTP(S) URL loading with hujson support

Example Test Pattern

func TestEntry_AddPrefix(t *testing.T) {
    tests := []struct {
        name    string
        cidr    any
        wantErr bool
    }{
        {"IPv4 CIDR", "192.168.1.0/24", false},
        {"IPv6 CIDR", "2001:db8::/32", false},
        {"net.IP", net.ParseIP("192.168.1.1"), false},
        {"netip.Prefix", netip.MustParsePrefix("10.0.0.0/8"), false},
    }
    // ...
}

212 tests, zero dependencies, zero failures.

Original prompt

Please write comprehensive unit tests for lib package and make sure the coverage rate is above 90%. Try not to use third-party Golang dependencies if possible.

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