fix(typing): preserve decorated function type signatures with ParamSpec

[gencurrent]

I encounter this issue with Pyrefly constantly: the declared return type of the decorated function / method does not match the expected return type, so I have to add # ... ignore ... comments.

The @cachier decorator erases all type information from decorated functions. Because cachier() has no return type annotation and no generic type variables, type checkers see every decorated function -- both its signature and return value -- as Any:

@cachier()
def my_func(x: int) -> str:
    return str(x)

reveal_type(my_func)      # Any
reveal_type(my_func(5))   # Any
This breaks IDE autocomplete, disables type error detection on wrong arguments, and forces downstream typed codebases to use stubs or type: ignore.

Changes
src/cachier/core.py: Add ParamSpec/TypeVar generics to cachier(), _cachier_decorator(), and func_wrapper so the original function signature is preserved through the decorator
tests/test_typing.py: 12 mypy-based tests covering sync, async, complex signatures, wrong-arg rejection, and decorator argument variants
tests/requirements.txt: Add mypy as a test dependency
After:


reveal_type(my_func)      # (x: int) -> str
reveal_type(my_func(5))   # str

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.95%. Comparing base (4602401) to head (e6ff4b1).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #376   +/-   ##
=======================================
  Coverage   99.95%   99.95%           
=======================================
  Files          16       16           
  Lines        2098     2100    +2     
  Branches      251      251           
=======================================
+ Hits         2097     2099    +2     
  Partials        1        1           

Flag	Coverage Δ
local	66.42% <100.00%> (+0.03%)	⬆️
mongodb	39.76% <94.44%> (+0.05%)	⬆️
postgres	41.85% <94.44%> (+0.05%)	⬆️
redis	44.33% <94.44%> (+0.05%)	⬆️
s3	41.04% <94.44%> (+0.05%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/cachier/core.py	100.00% <100.00%> (ø)

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