Add cache keys for combinations

[Ap4sh]

This adds _cache_key() to the finite Combinations parents so they can be used by cached constructions such as cartesian_product

The issue was that Combinations(...) parents define equality but are not hashable, and they also did not provide _cache_key(). That made cartesian_product([Combinations([1,2,3], 2), Combinations([1,2,3], 2)]) fail while building the cached Cartesian product parent

The change keeps the parents unhashable and only adds cache keys for the existing cache machinery. The key includes the concrete class, the multiset data, k when present, and the as_tuples output mode

Fixes #39004
Related to #19986

Checks:

• reproduced the failure on the current dev image before the patch
• ./sage -t --long src/sage/combinat/combination.py
• ./sage -tox -e relint -- src/sage/combinat/combination.py
• ./sage -tox -e rst -- src/sage/combinat/combination.py
• manual checks for set, multiset, fixed-length, and as_tuples combinations inside cartesian_product

📝 Checklist

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• The description explains in detail what this PR is about
• I have linked a relevant issue or discussion
• I have created tests covering the changes
• I have updated the documentation and checked the documentation preview

⌛ Dependencies

None

[github-actions]

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