fix(cli): guard mops check --fix against undefined moc stdout

[giorgiopiatti-caffeinated]

Summary

mops check --fix can crash the entire command with:

TypeError: Cannot read properties of undefined (reading 'split')
    at parseDiagnostics (.../dist/helpers/autofix-motoko.js:7:10)
    at autofixMotoko (.../dist/helpers/autofix-motoko.js:111:33)

autofixMotoko spawns moc … --error-format=json via execa and feeds result.stdout straight into parseDiagnostics, which calls stdout.split("\n"). Per execa's documented contract, result.stdout can resolve to undefined (not "") when output isn't buffered, so parseDiagnostics(undefined) throws. The throw is unhandled (the autofix call in check.ts runs before, and outside of, the guarded moc --check block), so it aborts the whole command — and any deploy pipeline invoking mops check --fix (e.g. via caffeine check --fix) surfaces it as backend check failed (exit 1).

Fix

• parseDiagnostics now returns [] for a missing/undefined stdout instead of calling .split on it. The autofix pass degrades gracefully (no fixes applied) and the authoritative mops check still runs and reports the real diagnostics.
• Added unit tests for parseDiagnostics covering undefined, empty, and mixed JSON/non-JSON input.
• Changelog entry under ## Next.

Test plan

• npm run check (tsc) passes in cli/
• npm test -- check-fix.test.ts -t parseDiagnostics — 3 new tests pass
• CI green

Made with Cursor

[christoph-dfinity]

Approving to move things along. @Kamirus maybe take a look when you get back, I personally would want to move the check out of the parse function.