Better error handling in assignTestPorts

[CraigMacomber]

Description

This code used to simply try and parse an empty string as JSON and give an unexpected end of JSON syntax error if there was some issue, like those caused by include-install-build-tools overwriting the pnpm version causing pnpm to try and install a different version which failed due to network isolation (Noted in #27537 and blocking #27509). Now it actually outputs the errors in such cases.

Reviewer Guidance

The review process is outlined on this wiki page.

[github-actions]

Hi! Thank you for opening this PR. Want me to review it?

Based on the diff (41 lines, 1 files), I've queued these reviewers:

• Correctness — logic errors, race conditions, lifecycle issues
• Security — vulnerabilities, secret exposure, injection
• API Compatibility — breaking changes, release tags, type design
• Performance — algorithmic regressions, memory leaks
• Testing — coverage gaps, hollow tests

How this works

• Adjust the reviewer set by ticking/unticking boxes above. Reviewer toggles alone don't trigger anything.

• Tick Start review below to dispatch the review fleet.

• After review finishes, tick Start review again to request another run — it auto-resets after each dispatch.

• This comment updates as new commits land; your reviewer selections are preserved.

• Start review

[Copilot]

Copilot reviewed 1 out of 1 changed files in this pull request and generated no comments.

[alexvy86]

LGTM