fix(cache): separate positional and keyword cache keys

[UditDewan]

Fixes #3045.

Calling a cached function with ("value", 1) as a positional argument and then with value=1 currently returns the first result for both calls. The cache key concatenates positional arguments and keyword pairs, so these distinct calls become indistinguishable.

Keep the function, positional arguments, and sorted keyword pairs in separate tuple fields. This preserves function isolation, keyword-order independence, and the existing locking behavior.

Validation on Python 3.13.7:

• The new regression test fails on the original implementation in both call orders: 2 failed, 24 passed.
• With the fix, PYTHONPATH=backend python -m pytest backend/tests/test_cache.py -q: 26 passed.
• Ruff lint and formatting checks on both changed files passed.
• git diff --check passed.

The test also repeats both calls and checks that the function executes only once per distinct input.

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Summary by cubic

Fixes #3045. Calling a cached function with ("value", 1) as a positional argument and then with value=1 previously returned the same result for both calls because both produced the same cache key. The key now stores the positional arguments and sorted keyword pairs in separate tuple fields, so these calls get distinct cache entries. The regression test covers both call orders and verifies the function runs only once per distinct input.

^{Written for commit 58f7696. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}