Avoid failing validation requests when response is redirect

[westonruter]

Summary

This fixes a regression introduced by ae36769 in #5296 whereby a validation request returned JSON error data if the response was not an AMP page. An exception is needed for when the response is not an AMP page and yet is an HTTP redirect. The \AMP_Validation_Manager::validate_url() follows redirects, so we should allow redirects to pass through.

I discovered this when trying to do wp amp validation run --limit=1 --include=is_date which resulted in:

Warning: Validate URL error (RENDERED_PAGE_NOT_AMP): The requested URL did not result in an AMP page being rendered. URL: https://wordpressdev.lndo.site/?year=2021

This fixes that problem by allowing the request for https://wordpressdev.lndo.site/?year=2021 to redirect to https://wordpressdev.lndo.site/2021/.

Also amends #6063 by undoing erroneous addition of is_callable() check in d0d2137.

Checklist

• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

[codecov]

Codecov Report

Merging #6069 (ca1196e) into develop (8dad6e3) will decrease coverage by 0.00%.
The diff coverage is 62.50%.

❗ Current head ca1196e differs from pull request most recent head f634910. Consider uploading reports for the commit f634910 to get more accurate results

@@              Coverage Diff              @@
##             develop    #6069      +/-   ##
=============================================
- Coverage      74.82%   74.81%   -0.01%     
- Complexity      5767     5771       +4     
=============================================
  Files            230      230              
  Lines          17486    17492       +6     
=============================================
+ Hits           13084    13087       +3     
- Misses          4402     4405       +3     

Flag	Coverage Δ	Complexity Δ
javascript	79.84% <ø> (ø)	0.00 <ø> (ø)
php	74.58% <62.50%> (-0.01%)	5771.00 <0.00> (+4.00)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ	Complexity Δ
includes/class-amp-theme-support.php	85.90% <57.14%> (-0.30%)	249.00 <0.00> (+5.00)	⬇️
src/Infrastructure/ServiceBasedPlugin.php	85.51% <100.00%> (ø)	56.00 <0.00> (-1.00)

[github-actions]

Plugin builds for f634910 are ready 🛎️!

• Download development build
• Download production build

[pierlon]

QA Passed

Running the command wp amp validation run --limit=1 --include=is_date no longer results in the warning "Validate URL error (RENDERED_PAGE_NOT_AMP)" from occurring, and the URL is validated:

Success: 0 crawled URLs have invalid markup kept out of 1 total with AMP validation issue(s); 1 URLs were crawled.
+--------------------------+-----------+---------------+
| Template or content type | URL Count | Validity Rate |
+--------------------------+-----------+---------------+
| date                     | 1         | 100%          |
+--------------------------+-----------+---------------+