fix(reviewer): handle Anki 26.09 '/_anki/js/mathjax.js'

[david-allison]

Note

Assisted-by: GPT-6

Purpose / Description

Anki 26.09 adds /_anki/js/mathjax.js, this is not handled by our current MathJax vendoring

Fixes

• Part of Integrate Anki 26.09 #21926

Approach

• Add a regression test
• Fix the new URL

How Has This Been Tested?

Unit tests added

Checklist

• You have a descriptive commit message with a short title (first line, max 50 chars).
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code
• UI changes: include screenshots of all affected screens (in particular showing any new or changed strings)
• UI Changes: You have tested your change using the Google Accessibility Scanner

[david-allison]

ViewerResourceHandlerTest > MathJax implementation is available at the backend URL FAILED
    org.opentest4j.AssertionFailedError: actual value is null ==> expected: not <null>
        at app//org.junit.jupiter.api.Assertions.assertNotNull(Assertions.java:345)
        at app//kotlin.test.junit5.JUnit5Asserter.assertNotNull(JUnitSupport.kt:48)
        at app//kotlin.test.AssertionsKt__AssertionsKt.assertNotNull(Assertions.kt:147)
        at app//kotlin.test.AssertionsKt.assertNotNull(Unknown Source)
        at app//kotlin.test.AssertionsKt__AssertionsKt.assertNotNull$default(Assertions.kt:145)
        at app//kotlin.test.AssertionsKt.assertNotNull$default(Unknown Source)
        at com.ichi2.anki.ViewerResourceHandlerTest.MathJax implementation is available at the backend URL(ViewerResourceHandlerTest.kt:54)