Fix TypedDict qualifier inheritance with same name

[JanEricNitschke]

Discovered through CPython 3.14 test_typing.py: https://github.com/JanEricNitschke/typing_extensions/actions/runs/17619859892/job/50062609198#step:7:44

Tests and code are directly taken from CPython 3.14.

Tests: https://github.com/python/cpython/blob/3.14/Lib/test/test_typing.py#L8616

Code: https://github.com/python/cpython/blob/3.14/Lib/typing.py#L3139

Except that i omitted the raising of the TypeError when overwriting a mutable key with a read-only key pending further discussion.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.38%. Comparing base (9215c95) to head (bbf9dff).

@@ Coverage Diff @@## main #677 +/- ## ========================================== + Coverage 97.36% 97.38% +0.01%  ========================================== Files 3 3 Lines 7680 7716 +36 ========================================== + Hits 7478 7514 +36  Misses 202 202 

Flag	Coverage Δ
3.10	89.04% <100.00%> (+0.05%)	⬆️
3.10.4	89.04% <100.00%> (+0.05%)	⬆️
3.11	88.28% <100.00%> (?)
3.11.0	87.51% <100.00%> (+0.05%)	⬆️
3.12	88.23% <100.00%> (+0.05%)	⬆️
3.12.0	88.21% <100.00%> (+0.05%)	⬆️
3.13	82.47% <100.00%> (?)
3.13.0	82.47% <100.00%> (+0.08%)	⬆️
3.14	78.91% <100.00%> (+0.07%)	⬆️
3.9	89.75% <100.00%> (+0.04%)	⬆️
3.9.12	89.75% <100.00%> (+0.04%)	⬆️
pypy3.10	88.88% <100.00%> (+0.05%)	⬆️
pypy3.11	88.14% <100.00%> (+0.05%)	⬆️
pypy3.9	89.59% <100.00%> (+0.04%)	⬆️

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Files with missing lines	Coverage Δ
src/test_typing_extensions.py	98.40% <100.00%> (+<0.01%)	⬆️
src/typing_extensions.py	93.91% <100.00%> (+0.02%)	⬆️

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[JanEricNitschke]

This should also fix #308