fix: render math through a MathJax v2 and v4 compatible helper

[feanil]

TL;DR - ORA's two MathJax calls use the v2 MathJax.Hub API, which MathJax v4 removed. openedx/edx-platform#33555 upgrades the platform to 4.1.2, and when it lands those calls throw and math stops rendering in ORA prompts and submitted responses. This routes them through a helper that works on both v2 and v4, so it can merge now rather than being sequenced against the platform upgrade.

What changed?

• New oa_mathjax.js exporting typesetMath(element). It prefers v4's MathJax.typesetPromise, falls back to v2's MathJax.Hub.Queue, and does nothing when the runtime provides no MathJax.
• oa_server.js::renderLatex and lms/oa_base.js::bindLatexPreview now call it instead of MathJax.Hub.Queue directly.
• Dropped the MathJax stub in oa_shared.js. It guarded on typeof window.MathJax === 'undefined', which is false under v4, so it never engaged where it was needed. The helper covers the no-MathJax case it existed for.
• New spec covering all four states: v4, v2, v4-shaped-but-not-ready, and absent.
• Rebuilt static assets, bumped to 7.1.2.

Testing Instructions

make test-js passes: 288 of 289, and the one failure is a pre-existing flake in the "unsubmitted assessments" specs that also fails on master (it moves between oa_self.js and oa_staff_area.js with Jasmine's random spec order). Note make test-js rather than npm test, since the fixtures come from the render-templates step.

The bug itself reproduces without the branch: with the global set to {}, which is what v4 presents, the old MathJax.Hub.Queue(['Typeset', MathJax.Hub, el]) raises TypeError: Cannot read properties of undefined (reading 'Queue'). The does not throw when MathJax is v4-shaped but not ready spec covers that state against the new helper.

To check it by hand, open an ORA with latex enabled and confirm math renders in the prompt, in the response preview, and in a submitted response, on a platform serving either MathJax version.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.46%. Comparing base (eee6b7b) to head (ce9044c).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2454   +/-   ##
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  Coverage   95.46%   95.46%           
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  Files         198      198           
  Lines       22842    22842           
  Branches     1551     1551           
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  Hits        21807    21807           
  Misses        780      780           
  Partials      255      255           

Flag	Coverage Δ
unittests	95.46% <100.00%> (ø)

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