test(cache): cover rejected remote cache uploads

[wan9chi]

Motivation

With the write policy in place from #809, the e2e cases can cover the uploads the public cache service rejects. A developer running --remote-cache=read-write outside GitHub Actions, or a job for a pull request, should still see their task succeed and be told why nothing was uploaded.

Changes

• pull_request_upload: a pull request's token doesn't satisfy the write policy. The upload gets 403, and the details show Write not permitted.
• upload_without_token: an upload from outside GitHub Actions has no token. It gets 401, and the details show Invalid credentials.

Both tasks succeed. The snapshots come from the file backend here. The backend switch later in this stack must leave them unchanged, which shows that the emulated policy matches the service.

Stacked on #809.

🤖 Generated with Claude Code

[github-actions]

fspy benchmark

linux

dynamic/launch             change  -1.63%  [-17.11% .. +16.10%]  overhead  +288.89%
dynamic/access             change  +0.40%  [ -9.30% ..  +9.19%]  overhead   +14.35%
dynamic/access-relative    change  +0.76%  [ -3.74% ..  +5.92%]  overhead   +67.07%
dynamic/access-contended   change  -0.39%  [-16.55% .. +21.97%]  overhead   +24.79%
static/launch              change  +0.31%  [-10.74% .. +11.94%]  overhead  +654.20%
static/access              change  +1.01%  [ -6.78% .. +15.30%]  overhead +1380.91%
static/access-relative     change  -1.18%  [-16.85% ..  +2.72%]  overhead +1897.96%
static/access-contended    change  +1.04%  [ -2.15% ..  +5.35%]  overhead +1548.95%

macos

dynamic/launch             change  -0.28%  [ -4.09% ..  +3.50%]  overhead  +244.07%
dynamic/access             change  -0.35%  [ -7.19% ..  +4.23%]  overhead    +3.37%
dynamic/access-relative    change  -0.50%  [ -4.40% ..  +2.01%]  overhead  +275.21%
dynamic/access-contended   change  -8.01%  [-25.35% ..  +7.95%]  overhead    +2.98%

windows

dynamic/launch             change  -0.38%  [ -2.82% ..  +3.74%]  overhead   +27.05%
dynamic/access             change  +0.00%  [ -0.93% ..  +1.52%]  overhead    +1.18%
dynamic/access-relative    change  -0.18%  [ -0.95% ..  +0.57%]  overhead    +1.13%
dynamic/access-contended   change  -0.53%  [ -3.10% ..  +1.60%]  overhead    +1.81%