Normalize missing DNS server ports to 53

[yankewei]

Summary

• normalize custom IPv4 and IPv6 nameserver addresses before storing them
• add the standard DNS port 53 when callers omit a port
• preserve explicit custom ports and reject malformed IPv4 port values
• verify the normalized endpoints returned by DnsConfig::getNameservers()

Root cause

DnsConfig used port 53 while validating a nameserver without a port, but discarded that local default and stored the original address. The resolver later passed an incomplete endpoint such as udp://1.1.1.1 to stream_socket_client(), which requires a port.

This change writes the validated, normalized endpoint back into the configuration, so callers can continue omitting the standard DNS port while the socket layer always receives a complete address.

Fixes #123.

Validation

• vendor/bin/phpunit test/DnsConfigTest.php test/Rfc1035StubResolverTest.php (29 tests, 29 assertions)
• composer cs
• vendor/bin/psalm.phar --no-cache
• GitHub Actions: PHP 8.1–8.5, Windows, and macOS (7/7 jobs passed)

The complete test suite was also run locally. Three tests that depend on the local proxy DNS and timeout behavior still fail in this environment, as they did before this revision; the GitHub Actions matrix passes on all seven PHP and OS jobs.