[Bugfix] Preserve original DPI in pickled figures to avoid HiDPI doubling (swev-id: matplotlib__matplotlib-23476)

[casey-brooks]

Summary

• preserve the figure's original DPI when restoring pickled figures
• reapply the stored DPI after recreating pyplot managers
• add a HiDPI pickle/unpickle regression test

Testing

============================= test session starts ==============================
platform linux -- Python 3.10.19, pytest-9.0.2, pluggy-1.6.0
rootdir: /workspace/matplotlib
configfile: pytest.ini
plugins: timeout-2.4.0, rerunfailures-16.1, xvfb-3.1.1, cov-7.0.0, xdist-3.8.0
collected 183 items

lib/matplotlib/tests/test_pickle.py .................................... [ 19%]
........................................................................ [ 59%]
........................................................................ [ 98%]
... [100%]

============================= 183 passed in 1.44s ==============================

swev-id: matplotlib__matplotlib-23476

Closes #28

[casey-brooks]

Test & Lint Summary

• LD_LIBRARY_PATH=/root/.nix-profile/lib:/nix/store/qipd93x9gjyiygqk673rd2ssnf8y7jj0-gcc-14.3.0-lib/lib:/nix/store/f8w1i7yisixb9hivzbk0l4ixmf67fjqr-gcc-14.3.0-libgcc/lib:/nix/store/62yvg9afjvmwa8z17arn4giqmjzg2xd2-glibc-2.40-66/lib PYTHONPATH=/workspace/matplotlib/lib MPLBACKEND=Agg .venv/bin/pytest lib/matplotlib/tests/test_pickle.py (183 passed, 0 failed)
• PYTHONPATH=/workspace/matplotlib/lib .venv/bin/flake8 lib/matplotlib/figure.py lib/matplotlib/tests/test_pickle.py (no issues)

[noa-lucent]

Looks good—thanks for covering the regression with a dedicated test.