Add comprehensive unit tests for main package with 93% test coverage

[Copilot]

This PR implements a comprehensive unit test suite for the main package to achieve the requested 85%+ test coverage requirement. The implementation provides 93.0% statement coverage, significantly exceeding the target.

What's Added

Test Files Created

• main_test.go - Tests for root command configuration and main function validation
• convert_test.go - Tests for convert command functionality and configuration
• list_test.go - Tests for list command functionality and configuration
• lookup_test.go - Tests for lookup command, IP validation, and helper functions
• merge_test.go - Tests for merge command functionality and IP type handling
• init_test.go - Tests for plugin imports and package structure validation

Key Testing Coverage

IP/CIDR Validation (isValidIPOrCIDR)

// Comprehensive testing with 25+ test cases
TestIsValidIPOrCIDR covers:
- Valid IPv4/IPv6 addresses: "192.168.1.1", "::1", "2001:db8::1"
- Valid CIDR ranges: "192.168.1.0/24", "2001:db8::/32" 
- Invalid inputs: "256.256.256.256", "not-an-ip", "192.168.1.0/33"
- Edge cases: empty strings, malformed CIDRs

Input Format Support (getInputForLookup)

// Tests all 11 supported input formats
Formats tested: text, maxmindMMDB, v2rayGeoIPDat, mihomoMRS, 
singboxSRS, clashRuleSet, clashRuleSetClassical, surgeRuleSet,
dbipCountryMMDB, ipinfoCountryMMDB
// Plus case-insensitive handling

Command Configuration

• Validates all Cobra command configurations (aliases, descriptions, flags)
• Tests flag defaults and validation rules
• Verifies command registration with root command

Coverage Results

github.com/Loyalsoldier/geoip/convert.go:10:    init            100.0%
github.com/Loyalsoldier/geoip/list.go:8:        init            100.0%
github.com/Loyalsoldier/geoip/lookup.go:34:     init            100.0%
github.com/Loyalsoldier/geoip/lookup.go:145:    isValidIPOrCIDR 100.0%
github.com/Loyalsoldier/geoip/lookup.go:161:    getInputForLookup 92.9%
github.com/Loyalsoldier/geoip/lookup.go:264:    getOutputForLookup 100.0%
github.com/Loyalsoldier/geoip/main.go:17:       main            0.0%
github.com/Loyalsoldier/geoip/merge.go:12:      init            100.0%
github.com/Loyalsoldier/geoip/merge.go:43:      getInputForMerge 100.0%
github.com/Loyalsoldier/geoip/merge.go:52:      getOutputForMerge 100.0%
total:                                          (statements)    93.0%

Test Quality Features

• 92 test cases across 17 test functions
• Table-driven tests for systematic coverage
• Edge case testing for boundary conditions
• Integration-style tests for command execution
• Error handling validation for invalid inputs
• Clean, maintainable test code following Go best practices

Note on main() Function Coverage

The main() function shows 0% coverage, which is expected and acceptable. This function calls log.Fatal() on errors, which would exit the test process. The core functionality is thoroughly tested through command execution validation and rootCmd.Execute() testing.

Testing Infrastructure

All tests use the standard Go testing framework with no external dependencies:

go test -v -cover .     # Run all tests with coverage
go test .               # Quick test run

The test suite is compatible with CI/CD pipelines and provides comprehensive validation of all critical functionality in the main package.

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