FIX Correct trailing slash for subsite in another domain

[GuySartorelli]

Fixes https://github.com/silverstripe/silverstripe-subsites/actions/runs/10278313787/job/28441800184 which consists of failures like the below:

Failed asserting that two strings are identical.
--- Expected
+++ Actual
@@ @@
-'http://two.mysite.com/'
+'http://two.mysite.com/'

As of silverstripe/framework 5.2.20, Controller::normaliseTrailingSlash() won't normalise trailing slash for absolute URLs where the domain doesn't match the current site domain.

This means if getting a link to a page on Subsite A from Subsite B, if the domains don't match, the trailing slash won't be normalised.

It's possible that it would be better to implement the updateIsSiteUrl() extension hook in an extension for Director and saying "yes that's a site URL" if it belongs to a domain of any subsite. I didn't attempt that because I don't know what sort of flow-on effects that might have. I'm especially hesitant to do that in a patch release.

Note that the same problem happened on Fluent, see tractorcow-farm/silverstripe-fluent#872
Most likely we'll want to implement the updateIsSiteUrl() extension hook for both modules in a minor release, but I want at least one other opinion before I do that. Either way, IMO this PR should be merged for a patch fix.

Issue

• Broken builds and merge-ups .github#290

[GuySartorelli]

@silverstripe/core-team if anyone has a sec to look at this and give an opinion about the updateIsSiteUrl() option for a more robust solution in a minor release, I'd be grateful.