Only parse and mess with the URI in \Drupal\Core\Utility\UnroutedUrlAssembler::buildExternalUrl() if we have to

Problem/Motivation

In \Drupal\Core\Utility\UnroutedUrlAssembler::buildExternalUrl() we rebuild external URLs from their parts unnecessarily. PHP has lots of PHPism in the way it processes URLs but external systems don't all run PHP so we should not futz with external URLs unless we have to.

Steps to reproduce

See #1464244: Rewrite as URL adding equals sign to end of url. and #3038774: Url only outputs the last value of a query parameter

Proposed resolution

Only parse external URLs if we have to change something. This approach could be further improved in future issues by using PHP 8.5's new URI or URL classes and then only really messing with stuff when we have to change the query string. To quote the PHP 8.5 URI RFC the problems here are caused by

Query string parsing is a fuzzy area, since there are no established rules for parsing

Therefore I think avoiding any manipulation of external URLs is the best way to avoid bugs.

Remaining tasks

None

User interface changes

External URLs are not manipulated when used as menu links.

Introduced terminology

N/a

API changes

None

Data model changes

None

Release notes snippet

N/a