Ensure init called in protocol subclasses

[JanEricNitschke]

Discovered through CPython 3.12 test_typing.py: https://github.com/JanEricNitschke/typing_extensions/actions/runs/17639167194/job/50121940225#step:7:36

Tests are from here:

• https://github.com/python/cpython/pull/27545
• https://github.com/python/cpython/pull/28121
• https://github.com/python/cpython/pull/28121

Code is from current CPython:

• https://github.com/python/cpython/blob/main/Lib/typing.py#L1843
• https://github.com/python/cpython/blob/main/Lib/typing.py#L2121

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 97.37%. Comparing base (9215c95) to head (0ec3920).

@@           Coverage Diff           @@
##             main     #678   +/-   ##
=======================================
  Coverage   97.36%   97.37%           
=======================================
  Files           3        3           
  Lines        7680     7706   +26     
=======================================
+ Hits         7478     7504   +26     
  Misses        202      202           

Flag	Coverage Δ
3.10	89.03% <100.00%> (+0.03%)	:arrow_up:
3.10.4	89.03% <100.00%> (+0.03%)	:arrow_up:
3.11	88.26% <100.00%> (?)
3.11.0	87.50% <100.00%> (+0.04%)	:arrow_up:
3.12	88.21% <100.00%> (+0.03%)	:arrow_up:
3.12.0	88.20% <100.00%> (+0.03%)	:arrow_up:
3.13	82.49% <96.66%> (?)
3.13.0	82.49% <96.66%> (+0.09%)	:arrow_up:
3.14	78.94% <96.29%> (+0.10%)	:arrow_up:
3.9	89.74% <100.00%> (+0.03%)	:arrow_up:
3.9.12	89.74% <100.00%> (+0.03%)	:arrow_up:
pypy3.10	88.86% <100.00%> (+0.03%)	:arrow_up:
pypy3.11	88.12% <100.00%> (+0.04%)	:arrow_up:
pypy3.9	89.57% <100.00%> (+0.03%)	:arrow_up:

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Files with missing lines	Coverage Δ
src/test_typing_extensions.py	98.40% <100.00%> (+<0.01%)	:arrow_up:
src/typing_extensions.py	93.87% <100.00%> (-0.02%)	:arrow_down:

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