fix: remove @edx/frontend-lib-learning-assistant dependency

[arbrandes]

Description

The @edx/frontend-lib-learning-assistant library has been fully deprecated and is now being removed. This PR removes the dependency along with all its usage across the codebase: the Redux reducer registration, the learningAssistant state selectors, the learning_assistant_enabled field mapping, related config variables (CHAT_RESPONSE_URL, ENABLE_XPERT_AUDIT), and associated test and pact contract references.

The ContentTools component previously read sidebarIsOpen from the learningAssistant Redux state to conditionally hide itself when the chatbot sidebar was open. With the library removed, content tools now render unconditionally.

Fixes #1798

Testing

When testing this via npm run dev you may run into loud SASS build warnings. That's a separate issue, fixed by openedx/frontend-build#683.

LLM usage notice

Built with assistance from Claude models (mostly Opus 4.6).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.26%. Comparing base (0664dc3) to head (72cc0a1).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1894      +/-   ##
==========================================
- Coverage   91.27%   91.26%   -0.01%     
==========================================
  Files         343      343              
  Lines        5766     5761       -5     
  Branches     1387     1385       -2     
==========================================
- Hits         5263     5258       -5     
  Misses        484      484              
  Partials       19       19              

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[arbrandes]

@jristau1984, hi! This is the PR that implements the DEPR, which by the timeline should be good to go (i.e, good to break). Objections?

[jristau1984]

@jristau1984, hi! This is the PR that implements the DEPR, which by the timeline should be good to go (i.e, good to break). Objections?

Feel free to proceed, from 2U perspective. Thanks!