feat(cache): use HTTP 404 for remote cache fetch misses

[wan9chi]

Motivation

The public cache service (#718) follows its RFC and answers a fetch that matches neither key with HTTP 404 and a plain-text body. It never sends kind: "not_found". The client and the Node test backend still used a 200 response with kind: "not_found", so a miss meant different things depending on the server. This switches both to 404 and drops the not_found kind, giving the client and both servers one miss contract.

Changes

• vt_remote_cache: Client::fetch returns Result<Option<Fetched>, Error>, with None for a 404 response. Fetched only describes the body of a 200 response, so its NotFound variant is removed. A 200 response with kind: "not_found" is now a malformed response. Every other non-200 status is still an error, and a 404 download still fails.
• Test backend (packages/tools): a fetch miss gets a 404 with the body Not found, logged as POST /fetch 404.
• The remote cache e2e snapshots change only in those backend lines and responses. The vp run output is unchanged.

[github-actions]

fspy benchmark

linux

dynamic/launch             change  -0.06%  [ -8.32% ..  +7.09%]  overhead  +267.01%
dynamic/access             change  +0.31%  [ -0.76% ..  +1.28%]  overhead   +11.92%
dynamic/access-relative    change  +0.11%  [ -1.57% ..  +1.14%]  overhead   +58.39%
dynamic/access-contended   change  -0.48%  [ -3.49% ..  +1.28%]  overhead   +13.79%
static/launch              change  +0.36%  [ -4.42% ..  +6.04%]  overhead  +736.12%
static/access              change  +0.20%  [ -0.87% ..  +1.47%]  overhead  +818.81%
static/access-relative     change  +0.09%  [ -0.72% ..  +0.91%]  overhead +1405.75%
static/access-contended    change  +0.17%  [ -0.33% ..  +0.68%]  overhead +3207.93%

macos

dynamic/launch             change  -0.58%  [ -2.98% ..  +2.38%]  overhead  +214.35%
dynamic/access             change  -0.69%  [ -3.44% ..  +2.15%]  overhead    +1.46%
dynamic/access-relative    change  +0.29%  [ -1.33% ..  +1.75%]  overhead  +273.03%
dynamic/access-contended   change  +1.26%  [ -2.39% .. +16.92%]  overhead    +2.35%

windows

dynamic/launch             change  -0.37%  [ -4.22% ..  +4.93%]  overhead   +24.19%
dynamic/access             change  +0.00%  [ -1.45% ..  +1.13%]  overhead    +1.32%
dynamic/access-relative    change  -0.18%  [ -1.97% ..  +1.48%]  overhead    +1.50%
dynamic/access-contended   change  -0.76%  [-13.87% ..  +4.25%]  overhead    +2.88%