Add unit tests for info_* helper functions with mocked subprocess and file I/O

[Copilot]

Addresses review feedback requesting test coverage for the info_* helper functions introduced in #113. These functions gather system information via subprocess calls and /proc reads but lacked unit tests.

Changes

• File I/O tests: Mock /proc/version and /proc/uptime reads for info_kernel_version() and info_uptime_seconds() with exception handling

• IPv4/IPv6 detection tests: Mock subprocess.run for all three fallback methods in info_default_ipv4() (dig → curl → ip command) and info_default_ipv6() with early-exit validation

• Update checking tests: Mock subprocess calls to /usr/libexec/rpcd/ns.update for info_package_updates_available() and info_image_updates_available()

• Version parsing tests: Validate parse_version() handles prefix removal, comparison operators, and invalid input

All tests use unittest.mock.patch and mock_open to isolate system dependencies. 17 new tests covering success paths, fallback chains, and failure modes.

def test_info_default_ipv4_curl_fallback(tmp_path):
    """Test info_default_ipv4 falls back to curl when dig fails"""
    u = EUci(confdir=tmp_path.as_posix())
    
    def mock_subprocess_run(cmd, **kwargs):
        result = MagicMock()
        if 'dig' in cmd:
            result.returncode = 1
        elif 'curl' in cmd:
            result.returncode = 0
            result.stdout = '203.0.113.1\n'
        return result
    
    with patch('subprocess.run', side_effect=mock_subprocess_run):
        result = inventory.info_default_ipv4(u)
        assert result.startswith('anon-')

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