test(coverage): cover the coverage-run CLI's non-Error rejection fallback

[hivecommons-hive]

Test Improvement

Adds one case to tests/e2e-coverage-run.test.mjs covering the ?? error
fallback in the CLI entrypoint of tests/tools/e2e-coverage-run.mjs:

209 main().catch((error) => {
210   console.error(error.stack ?? error);   // line 211
211   process.exitCode = 1;
212 });

The existing sibling case (tests/e2e-coverage-run.test.mjs:229) spawns the
tool with a bogus command, where main() throws new Error(...). That covers
the error.stack side and left the fallback beside it as the file's only
uncovered region.

Why the fallback matters. .github/workflows/ci.yml:231 and :248 run this
tool as a command, and its stderr is the only account the End-to-end coverage
job gives of why it stopped. A rejection that is not an Error has no .stack;
without the fallback that log would read undefined and nothing else.

How the case reaches it. Every throw in the tool is new Error(...), so
main() never rejects with a stackless value on its own. A --import prelude
makes the one host call main() issues on its success path — the
process.stdout.write() of the manifest — throw a bare string, so the rejection
arrives at the entrypoint in exactly the shape a non-Error would. The init
itself succeeds first, which keeps the case about the reporting path rather than
about the command; the test asserts that by reading the manifest back.

The case asserts exit status 1, that stderr carries the thrown value, and
explicitly that stderr does not contain undefined — the failure mode the
fallback exists to prevent.

Verification

TZ=UTC node tests/tools/coverage-report.mjs (node v26.10.0), run locally on
this branch:

	before	after
tests/tools/e2e-coverage-run.mjs	100.00 | 98.65 | 211	100.00 | 100.00

npm run test:unit:coverage:check exits 0 on this branch. No threshold in
package.json:70 is changed, and no workflow file is touched.

Scope

One new test case in one existing file. It does not edit
tests/tools/e2e-coverage-report.mjs or tests/e2e-coverage-report.test.mjs
(#1163), tests/tools/e2e-data-fixtures.cjs,
tests/e2e/fixtures/data-variants/metrics.json or
tests/e2e-data-fixtures.test.mjs (#1171),
tests/architecture-content-mirror.test.mjs (#1165), scripts/audit-gate.mjs
(#1161), or CONTRIBUTING.md (#1166).

Related Issue

Closes #1173

— hive: agent=quality backend=copilot model=claude-opus-5 copilot=1.0.88

[hivecommons-hive]

Important

Held for human review by the hive's ACMM level gate.

This PR was opened by the "quality" agent while Hive policy required a human checkpoint for that agent. Non-outreach agents are held at ACMM L3–L5; the outreach agent is always held because it publishes project-facing communication.

Hive will keep the hold label until a human removes it. Operators can make a deliberate one-off release during an ACMM level change with release_level_holds=true, but level changes never release this hold automatically.