fix: upload_systest_dep.sh: ignore-content-length

[basvandijk]

Problem

//rs/tests/nested:sev_recovery (and potentially any Farm-based systest) can
fail during setup with what looks like a bazel-remote connectivity error:

Found dep to upload ic-os/guestos/envs/sev-recovery-dev/disk-img.tar.zst (66d0…)
curl: (8) Invalid Content-Length: value
Failed to reach bazel-remote at 'http://server.bazel-remote.svc.cluster.local:8080/cas/66d0…'

Example run: https://github.com/dfinity/ic/actions/runs/28751952218/job/85252526147

Root cause

It is not a reachability problem. upload_systest_dep.sh's dep_in_cache()
probes bazel-remote with a HEAD request and reads only the HTTP status code:

code=$(curl --silent --show-error --max-time 30 -o /dev/null -w '%{http_code}' --head "$url")

bazel-remote's HEAD handler answers with, unconditionally:

ok, size := h.cache.Contains(r.Context(), kind, hash, -1)
...
w.Header().Set("Content-Length", strconv.FormatInt(size, 10))

Contains() returns "the size if known (or -1 if unknown)". A blob found in
the local disk cache always has a real size — which is why the two dev
images just above returned 200 (already uploaded). But on a local miss
Contains() falls through to the proxy/upstream backend, which can report a
blob as present but with unknown size (-1). The SEV-recovery disk image was
only in the proxy backend on that runner's cluster, so bazel-remote emitted the
literal header Content-Length: -1.

curl treats a negative Content-Length as a protocol violation and aborts with
exit code 8 (Invalid Content-Length: value) before printing %{http_code}.
The if ! code=$(curl …) branch then fires, the script prints
Failed to reach bazel-remote… and exits 1, failing the test setup. Because it
depends on whether the blob happens to be proxy-only vs. locally cached on a
given runner, the failure is intermittent.

Fix

Pass --ignore-content-length to the HEAD probe. curl then skips parsing the
bogus header and returns the real status code (200), so an already-cached dep
is correctly detected as uploaded instead of being misreported as an unreachable
server.

[Copilot]

Pull request overview

This PR deflakes Farm-based systest setup by making the bazel-remote cache probe robust to invalid Content-Length headers returned on HEAD responses when artifacts exist only in the proxy/upstream backend.

Changes:

• Add --ignore-content-length to the curl --head probe in dep_in_cache() so curl still returns the HTTP status code even when bazel-remote responds with Content-Length: -1.
• Document the underlying bazel-remote behavior and why the flag is required to avoid misclassifying cached artifacts as connectivity failures.

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