ci: re-add robot_descriptions caching, keyed by python-version this time

[petercorke]

Thanks for contributing to RTB!

Summary

PR #530 added actions/cache steps for ~/.cache/robot_descriptions, but they were removed in PR #536: robot_descriptions wasn't even a declared dependency at the time, so every run hit ModuleNotFoundError before any clone was attempted — the caching solved a problem that didn't exist yet, and had its own bug (cache key was OS-only, so it couldn't help across the os x python-version matrix's parallel jobs anyway).

That dependency gap is long fixed, and robot_descriptions is now genuinely exercised (YuMi, Valkyrie, UR3/5/10, Jaco, PR2, ...). Hit a concrete instance of the predicted failure mode directly: a real git-clone timeout in the macos-latest/3.12 job of PR #611's first CI run (unrelated to that PR's own change — just an ordinary transient network flake against robot_descriptions' upstream asset repos).

Re-adding caching with the corrected key (now includes python-version) to test-core, test, and coverage — and also docs-build, which wasn't covered by the original PR but genuinely needs it too: intro.rst's YuMi example runs as a live .. runblock:: during the Sphinx build.

Related issue

Checklist

Only the first item below is checked automatically — the rest are a self-check for you before requesting review, nothing currently verifies them for you.

• PR title follows Conventional Commits (type: description) — checked automatically, see the "Check PR title" status
• Tests pass locally (pytest)
• Added/updated tests for this change, if applicable
• New/changed code is type-hinted with modern syntax (X | Y, list[X], not Union/Optional/List)
• Docstrings updated (reST style: :param:, :returns:; type hints in the signature cover types now, :type:/:rtype: are rarely needed)
• PR is as small/focused as practical — if it tackles several unrelated things, consider splitting it so each can be reviewed and accepted independently
• No test files, data files, or notebooks specific to your own project — PyPI has strict package size limits, and RTB is already split into a toolbox and a data package. Notebooks, if of general interest, should have output cleared before committing.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (5977ac4) to head (fe7ca56).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #612   +/-   ##
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  Coverage   0.00%   0.00%           
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  Files        142     142           
  Lines      13898   13898           
=====================================
  Misses     13898   13898           

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