feat(python): type and document explicit cache key selectors

[lan17]

Python's docs currently lead with inferred id_arg keys even though cache_key already supports TypeScript-style explicit {id, args} selection. Make that existing selector the primary documented style and export CacheKeySpec / KeyScalar for typed selectors. Preserve decorated functions' parameter and result types for synchronous, asynchronous, and custom-awaitable loaders, including use with asyncio.create_task.

Keep id_arg, adapters, and ignored arguments supported. Explicit selectors still receive the original positional and keyword arguments; loader defaults are deliberately not injected because that could break callbacks or change existing keys. Document matching selector defaults and method self, and cover these conventions with runtime and consumer type-check regressions. Cache execution and Redis wire behavior are unchanged.

Validation:

• make check-python: 2,228 passed, including focused mypy consumer checks and committed conformance smoke histories.
• make integration-python: 343 passed against Redis and 343 against Valkey, including Cluster, invalidation vectors, and the changed Redis documentation example.
• make docs and make audit: passed.
• Python wheel/sdist build and isolated installation smoke checks: passed.
• Ruff on changed Python files and independent source review: clean.

— Levicus 🤖

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.73%. Comparing base (bb27803) to head (0680d61).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #214   +/-   ##
=======================================
  Coverage   93.73%   93.73%           
=======================================
  Files          78       78           
  Lines       11351    11357    +6     
  Branches      657      656    -1     
=======================================
+ Hits        10640    10646    +6     
  Misses        641      641           
  Partials       70       70           

Flag	Coverage Δ
go	91.17% <ø> (ø)
python	92.08% <100.00%> (+0.03%)	⬆️
rust	94.29% <ø> (ø)
typescript	96.56% <ø> (ø)

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