FIX Director::hostName() returning null for bare hostnames

[erikfrerejean]

Description

Director::hostName() returns null when the resolved host is a bare hostname (no port, no protocol) — for example, the RFC 7230-compliant Host header value example.com. PHP's parse_url() interprets such a string as a path and returns null for PHP_URL_HOST, so the wrapping ?: null propagates that null upwards and consumers receive null instead of the expected hostname.

The fix delegates to protocolAndHost() so parse_url() always receives a scheme-prefixed URL, which it can parse reliably regardless of whether the host carries a port or not.

The bug was reported (#11081) and reproduces verbatim against the current 6.2 branch.

Manual testing steps

1. Configure a Silverstripe project so that Director::host() resolves via $_SERVER['HTTP_HOST'] (i.e. unset alternate_base_url / no trusted-proxy override).
2. Send a request where the Host header is a bare hostname without a port, e.g. Host: example.com.
3. Before this PR: Director::hostName() returns null. After this PR: it returns "example.com".

The included unit test (DirectorTest::testHostNameWithoutPort) reproduces this without needing a live server.

Issues

• Incorrect hostname lookup when relying on $_SERVER['HTTP_HOST'] #11081

Pull request checklist

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