Test oai-spec-test package checkout layout

[handrews]

Ensure oai-spec-test works in developer mode. Existing tests for it use the installed package layout.

[miqui]

@handrews

my review:

Verified:

• Checked out the branch, ran npm install + npx vitest run — all 10 test files / 25 tests pass, including the new one.
• The fixture is correctly targeted: bin/oai-spec-test resolves c8/vitest via createRequire(import.meta.url), so Node's resolution walks up from the script's own directory — this test's root/node_modules fixture is genuinely a different resolution path than the nested-consumer fixture in bin-resolution.test.mjs, so it's not redundant coverage.
• Assertions check both the c8 --100 invocation and the vitest ... run --coverage --runInBand invocation, which is what actually distinguishes this from a broken resolution path.
• Temp-dir cleanup via afterEach matches the pattern already used in the other 6 test files with this same boilerplate (bin-resolution.test.mjs, release-commands.test.mjs, etc.) — no new duplication pattern introduced, just following existing convention.

findings: none, well put together.

[miqui]

@handrews repo perms, can't provide a PR review.