fix: Preserve decorated symbol types in docs_group and docs_name

[vdusek]

The docs_group and docs_name decorators in apify._utils were typed Callable -> Callable, which erased the types of every decorated public API symbol — Configuration, ProxyConfiguration, Webhook, the charging classes, event models, and ApifyRequestList all revealed as Unknown/Any to users' type checkers (Actor survived only thanks to an explicit cast). Both decorators are now identity-typed as Callable[[T], T], reusing the module's existing bound TypeVar and matching how Crawlee fixed the same bug.

Also adds a regression test: a runtime identity check plus a type-level guard that relies on the unused-ignore-comment = "error" ty rule — if the decorators ever erase types again, the guard's suppressions become unused and uv run poe type-check fails.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.82%. Comparing base (a87e8d1) to head (33a9159).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #964      +/-   ##
==========================================
- Coverage   90.00%   89.82%   -0.19%     
==========================================
  Files          49       49              
  Lines        3053     3085      +32     
==========================================
+ Hits         2748     2771      +23     
- Misses        305      314       +9     

Flag	Coverage Δ
e2e	35.98% <100.00%> (+0.31%)	⬆️
integration	56.95% <100.00%> (+0.09%)	⬆️
unit	78.63% <100.00%> (-0.08%)	⬇️

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