This PR fixes an issue in isValidHost where setting the devServer.host option could unintentionally block otherwise valid hostnames.

Currently, when validateHost is true, the code combines IP checks, localhost checks, and this.options.host into a single expression. This makes the logic harder to reason about and causes unexpected rejections when a custom host is configured.

This change makes the behavior explicit and correct:

• Always allow:

  • IPv4 and IPv6 addresses

  • localhost and *.localhost

• Only allow this.options.host when validateHost is enabled

This preserves the existing security model while fixing cases where valid requests were rejected when using a custom host.

Why this is needed

When users configure devServer.host, they expect it to be accepted consistently. The previous logic could cause valid requests to be rejected depending on validateHost, which is confusing and not intended behavior.

This change aligns the implementation with the documented behavior and avoids unexpected host blocking.

Fixes #5603