fix: The new SeparateBodyFileCache design makes updates/reads non-atomic (#324)

[princ0301]

Fixes #324

Summary

Fixes non-atomic updates/reads for SeparateBodyFileCache by ensuring the body is written before metadata. On read, it now verifies that both metadata and body files are present, treating an entry as invalid if the body is missing.

Validation

Tests passing after 1 attempt(s).

tests/test_expires_heuristics.py ................                        [ 59%]
tests/test_max_age.py ..                                                 [ 61%]
tests/test_redirects.py ....                                             [ 65%]
tests/test_regressions.py ..                                             [ 67%]
tests/test_serialization.py ..........                                   [ 77%]
tests/test_server_http_version.py .                                      [ 78%]
tests/test_storage_filecache.py ..................                       [ 96%]
tests/test_storage_redis.py ...                                          [ 99%]
tests/test_vary.py .                                                     [100%]
============================= 101 passed in 2.98s ==============================