Remove Task type with pool now calling Do directly

[alesr]

The Task[T] struct and its Fn func(T) field were a design inconsistency. the Input interface constraint required every type to implement Do(context.Context), but the pool never called it.

Instead, we were only using it as a generic constraint and users had to wrap their Do call inside a redundant Fn closure, making the API annoying to use.

This PR fixes that by removing Task[T] entirely. The pool now calls input.Do(ctx) directly, so the Input constraint is fully honoured.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.07%. Comparing base (99d863e) to head (62e7fbe).

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #2      +/-   ##
==========================================
+ Coverage   86.27%   96.07%   +9.80%     
==========================================
  Files           1        1              
  Lines          51       51              
==========================================
+ Hits           44       49       +5     
+ Misses          6        2       -4     
+ Partials        1        0       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.