Scope ETag cache to REST client and bound it by bytes

Restrict the conditional-request cache to the long-lived local (stdio)
REST client and give the raw-content client a separate transport without
it, so large file bodies are streamed rather than buffered into memory.

Bound the cache by a per-entry and total-byte budget in addition to the
entry count, and never retain responses marked non-storable by HTTP
cache directives (request or response Cache-Control: no-store, or
Vary: *).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 4959e1f9-f8e6-4e97-a487-f395a0123c79