Type-check all values for duplicate TypedDict keys

[Hmanbo]

Fixes #21841.

Summary

• Preserve every explicitly written value for a duplicate TypedDict key so
  mypy checks each expression that Python evaluates.
• Keep the final explicit value first for generic TypedDict inference while
  excluding synthetic TempNode values from overwritten dictionary unpack
  entries.
• Add regression coverage for constructors, contextually typed dictionary
  literals, and duplicate keys followed by a dictionary unpack.

Tests

.venv\Scripts\python.exe -m pytest -n0 -k testTypedDictDuplicateKeysTypeCheckAllValues
.venv\Scripts\python.exe -m pytest -n0 mypy/test/testcheck.py::TypeCheckSuite::check-typeddict.test
.venv\Scripts\python.exe runtests.py self
.venv\Scripts\pre-commit.exe run --files mypy/checkexpr.py test-data/unit/check-typeddict.test
git diff --check

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅