Bump test coverage, revise absorption rester, remove deprecated methods

[esoteric-ephemera]

• Adding test coverage for the client + core functionality
• Removing deprecated methods which no longer have planned use in the client
• Revise the absorption rester to be consistent with its API spec, and add tests

[codecov-commenter]

Codecov Report

❌ Patch coverage is 94.36620% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.04%. Comparing base (a8210aa) to head (6ff1e3c).

Files with missing lines	Patch %	Lines
mp_api/client/routes/materials/absorption.py	77.77%	2 Missing ⚠️
...pi/client/routes/materials/electronic_structure.py	94.28%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1042      +/-   ##
==========================================
+ Coverage   66.83%   74.04%   +7.20%     
==========================================
  Files          50       50              
  Lines        2801     2793       -8     
==========================================
+ Hits         1872     2068     +196     
+ Misses        929      725     -204     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[esoteric-ephemera]

@tschaume no need to look at this now - going to merge this in so it can unblock some work on @kbuma's side. The only changes to the client are rewriting the absorption rester so it works, and removing some deprecated serde's

If we need to restore anything prior to this down the line, happy to!