fix(@angular/ssr): allow underscores in host validation

The VALID_HOST_REGEX has been updated to include the underscore
character. This ensures that hostnames containing underscores (e.g., in
some internal or development environments) are correctly validated
instead of being rejected.

Closes angular#32835

(cherry picked from commit e7e434c)

Author: alan-agius4

packages/angular/ssr/src/utils/validation.ts

@@ -24,7 +24,7 @@ const VALID_PROTO_REGEX = /^https?$/i;
 /**
  * Regular expression to validate that the host is a valid hostname.
  */
-const VALID_HOST_REGEX = /^[a-z0-9.:-]+$/i;
+const VALID_HOST_REGEX = /^[a-z0-9_.-]+(:[0-9]+)?$/i;
 
 /**
  * Regular expression to validate that the prefix is valid.