#3378 verification: run the upgrade-path test in parallel, and skip the dynamic test for no-op bumps

Found while verifying #3325, which bumped 697 extensions. verify-build's upgrade-path-test (scripts/check_upgrades.ts --path-test) ran for 99 minutes, because the loop over bumped extensions (scripts/check_upgrades.ts, the for loop over bumpedByExtension) tests one extension at a time: pull the published version, create an instance, swap in the local source, upgrade. Every regenerate that bumps many extensions pays the same cost, and #3325 needed three verification rounds, roughly five hours of it.

Change

• Run the per-extension path tests with bounded concurrency (8 to 16), each in its own temp repo, as each already is. Keep every extension's output grouped, so a failure stays readable, and keep the error list and exit code the same.
• Optionally: when an extension's only upgrade entries are no-op ones (description No schema changes, identity upgradeAttributes), replace the dynamic test with a check that the published version still pulls and imports. The static check already proves the last upgrade entry matches the new version. The pull is what found the broken published aws/chime, glue and transfer (#3367).
• Keep the rule that any registry failure other than not found fails the gate.

Notes

• scripts/ and verification/ are trust-root paths; this goes through the Review Integrity audit. It could share a PR with #3349.
• Cover the concurrency in scripts/check_upgrades_test.ts: results and errors match a sequential run, and output stays grouped.