Use go test -json in updateFailing, detect baseline errors

[jakebailey]

This does two things:

• Uses go test -json to stream the output in a parseable format.
• With the parsed output, read each failing test's output to see if all of its failures are due to missing baselines.

We have no tests where this matters (I or others had fixed them previously), but I have a change where it does matter in the future.

[Copilot]

Pull request overview

This PR modernizes the updateFailing script by switching from synchronous test execution with regex parsing to streaming JSON output from go test -json. The key improvement is the ability to distinguish between baseline-only failures (which shouldn't mark tests as failing) and genuine test failures, enabling more accurate tracking of truly broken tests.

Key changes:

• Migrated from execFileSync to spawn with streaming JSON output parsing
• Added logic to differentiate baseline failures from real test errors by analyzing test output
• Simplified the CI workflow by removing redundant test and baseline-accept steps

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 5 comments.

File	Description
internal/fourslash/_scripts/updateFailing.mts	Completely refactored from synchronous regex-based parsing to async streaming with JSON parsing, added baseline-only failure detection logic
.github/workflows/ci.yml	Removed redundant baseline-accept and test steps that are now handled more efficiently by the updated script

[jakebailey]

I need to think about this harder; I have to go manually merge this into one of my branches to get it to test so it's annoying to figure out.

[jakebailey]

No, this does seem to work, I just merged the two PRs together wrong.