* refactor: Extract shared cooldown helpers to package-level functions
* feat: Add PyPI cooldown handler with PEP 691 file parsing
* feat: Add PyPI cooldown file stripping logic
* feat: Implement PyPI cooldown HandleMetadataRequest with PEP 691 filtering
* feat: Wire PyPI cooldown into pypi_registry interceptor
* update headers for no cache
* fix: Strip conditional GET headers to prevent 304 bypass in cooldown handlers
pip and npm clients cache Simple API / registry responses with ETags. On
subsequent requests they send If-None-Match, which causes the server to
return 304 Not Modified with no body. The cooldown response modifier
received an empty body, failed to parse it, and failed-open — letting
the client use its stale cached (unfiltered) response.
Fix: delete If-None-Match and If-Modified-Since from the request before
forwarding, forcing a full 200 response so the modifier always has a
body to filter.
Also removes the Content-Type guard from the PyPI modifier (the empty
Content-Type on 304 responses was a symptom of the same root cause) and
replaces Cache-Control: no-cache with the more targeted header deletion.
* docs: Add PyPI cooldown limitation for pip < 22.3 to dependency-cooldown docs
