test: lake: mock remote cache artifact transfers

[tydeu]

This PR adds a Lake test which performs mock artifact transfers to/from a remote cache service using a mock Python HTTP server. This serves as an alternative to the existing online cache test which covers full integration with Reservoir but also requires secrets and network access and is thus ill-suited for CI.

This PR also fixes test_err in the Lake test harness, which did not fail when an Lake invocation unexpectedly succeeded. All tests still pass, so no failures were obscured by this issue.

Test Cases

• cache put, put-staged, and stage, plus rejected artifact and outputs uploads
• cache get by --scope and by --repo, --force-download, --mappings-only, revision search and --max-revs, and platform- and toolchain-partitioned revision paths
• download failures: a missing artifact, a content hash mismatch, a body cut short of its declared Content-Length, a connection closed with no response, a response carrying no body at all, and an error response whose body is not an artifact
• misreported transfers: a curl that reports no transfer while claiming success, one that reports failure for artifacts that arrived complete, and one that reports success for artifacts it never wrote
• on-demand artifact fetches during a build, over both bucket and Reservoir URLs
• the Reservoir batch URL lookup, including a short result list and an API error object
• artifacts that share a content hash across extensions, which download once and are copied, not hard linked, into place
• Lake: one unreadable artifact aborts the whole lake cache get #14698: a batch in which one artifact yields no output file must report "failed to download some artifacts" and retain the artifacts that did transfer, rather than abort on the missing file.

Failure case also assert exactly which artifacts remain in the local cache, and those that could poison it assert that a subsequent good fetch still succeeds.

🤖 Prepared with Claude Code

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI can not be attempted yet, as the nightly-testing-2026-08-05 tag does not exist there yet. We will retry when you push more commits. If you rebase your branch onto nightly-with-manual, reference manual CI should run now. You can force reference manual CI using the force-manual-ci label. (2026-08-06 04:43:41)

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ✅ Mathlib branch lean-pr-testing-14700 has successfully built against this PR. (2026-08-06 05:39:16) View Log
• ✅ Mathlib branch lean-pr-testing-14700 has successfully built against this PR. (2026-08-06 06:40:00) View Log