Add correctly named getCustomerRegisterUrl() and deprecate the typo (#36735)

[lbajsarowicz]

Description

Magento\Customer\Block\Account\AuthenticationPopup::getCustomerRegisterUrlUrl() has a doubled "Url" in its name. It is still there on 2.4-develop (AuthenticationPopup.php:126).

The method is public on an @api block, so it cannot simply be renamed. This adds a correctly named getCustomerRegisterUrl(), has the old method delegate to it, and marks the old one @deprecated with a @see pointing at the replacement.

This PR continues #36794 by @Vasudev-22 (Partner: EY), rebased onto current 2.4-develop. Their commits and authorship are preserved.

The one review point that was never resolved

@Den4ik and @hostep both reviewed the original PR and every point was addressed except this one, from @hostep:

I think this one should be public instead of private, because the original method was also public? [...] won't we cause confusion with callers of the original method when we mark that one as deprecated and give no new method as alternative to call?

I agree, and this PR makes the replacement public. Deprecating a public method while offering only a private replacement leaves external callers with nowhere to migrate, which defeats the purpose of the deprecation. Adding a public method is additive; it is renaming or removing one that breaks backward compatibility.

Fixed Issues

Fixes #36735

Manual testing scenarios

1. Open any storefront page with the authentication popup enabled.
2. Confirm the "Create an Account" link in the popup still resolves to customer/account/create.
3. Call both getCustomerRegisterUrl() and the deprecated getCustomerRegisterUrlUrl() on the block and confirm they return the same URL.

Questions or comments

Note for the Semantic Version Checker build: this adds a public method to a class annotated @api @since 100.0.2. That is an additive (minor) change rather than a breaking one, but it will show up in the SVC report, so flagging it explicitly rather than letting it surprise a reviewer. No existing signature is changed and the deprecated method is retained.

Gates run locally on 2.4-develop (Warden, PHP 8.3):

• Unit: Customer/Test/Unit/Block/Account/ — 33 tests pass, including a new case asserting the deprecated method and its replacement return the same URL.
• PHPCS Magento2: clean. PHPStan level 1: no errors.
• Negative check on the new test: red against the unpatched block, green with the fix.

git grep across the repository finds no remaining caller of the old name outside its own declaration; the block's getConfig() was already updated to the new name in @Vasudev-22's original commits.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds are green)

[lbajsarowicz]

@magento run all tests

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[lbajsarowicz]

@magento run Static Tests

[hostep]

LGTM, @lbajsarowicz: static tests failure is probably on that copyright header: * Copyright 2015 Adobe. where the . at the end should be removed...