test: probe the provider sandbox instead of failing on it

The 16 sandboxed provider tests in `tests/unit/ci-artifact-contract.test.ts`
fail wherever unprivileged user namespaces are denied. bubblewrap is installed
and answers `--version`, but `bwrap --unshare-net … -- /bin/true` exits 1 with
`bwrap: loopback: Failed RTM_NEWADDR: Operation not permitted`, and it fails the
same way with no netns flag at all (`setting up uid map: Permission denied`), so
this is the whole nested-userns capability and not one option. Sixteen
assertion errors on every local run buried whatever else the file had to say.

`scripts/lib/provider-sandbox-probe.ts` now runs a trivial command under the
real isolation options — `runProviderInSandbox` spreads the same
`PROVIDER_SANDBOX_ISOLATION_ARGUMENTS`, and a test fails if an isolation option
is added to the run without the probe having to clear it. Capability is
measured, never inferred from the binary existing or from a version string;
both would pass here.

An unusable sandbox means two different things in two places, so the decision
is explicit. Locally it is an environment fact: the affected tests skip and
carry the bwrap diagnostic as their skip note, visible as `↓ … [reason]`. In CI
it is a regression — a security gate that silently stopped running is exactly
what these tests exist to catch — so the same probe result fails the run
through one guard test that says "the provider sandbox is unavailable" instead
of sixteen assertion errors.

CI is detected with `CI === "true"` via a new `isCiRun`, sharing the predicate
that already gates `ciBuildEnvironmentFailures`. The workflow sets it at the
top-level `env:` block, so it holds in every job; `CI_RUN_ID` and its
`GITEA_`/`GITHUB_` fallbacks are declared only by the release-tier provider
jobs and are absent from the merge gates that run this file, so keying on them
would have left the CI branch permanently dead.

The three `it.each` groups become `it.for` because only `.for` passes the test
context, which is what carries the skip note.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

scripts/run-and-validate-provider.ts

@@ -30,6 +30,7 @@ import {
   type ProviderGuardianLease,
 } from "./lib/provider-guardian-client.ts";
 import { createProviderOutputLimiter } from "./lib/provider-output-limiter.ts";
+import { PROVIDER_SANDBOX_ISOLATION_ARGUMENTS } from "./lib/provider-sandbox-probe.ts";
 
 const kind = process.argv[process.argv.indexOf("--kind") + 1];
 const PROVIDER_TMP_BYTES = 16_777_216;
@@ -246,10 +247,7 @@ async function runProviderInSandbox(
 ): Promise<void> {
   const cpuSeconds = providerCpuSeconds();
   const bwrapArguments = [
-    "--die-with-parent", "--new-session", "--as-pid-1",
-    "--unshare-pid", "--unshare-ipc", "--unshare-uts", "--unshare-net",
-    "--dev", "/dev", "--remount-ro", "/dev",
-    "--proc", "/proc", "--remount-ro", "/proc",
+    ...PROVIDER_SANDBOX_ISOLATION_ARGUMENTS,
     "--size", String(PROVIDER_TMP_BYTES), "--tmpfs", "/tmp",
     "--dir", "/tmp/provider-home",
     "--size", String(PROVIDER_MASK_BYTES), "--tmpfs", "/etc",