chore(build): remove redundant JavaScript pipeline error handler - #6985

Summary

Remove the redundant error handler from jsPipeline. The handler sat on the sourcemaps.init() stream, logged the error, and emitted end. It never covered the minifier, and Gulp 5 already reports failures from every stage and exits nonzero, so the handler only duplicated the error line. Four lines deleted, no other change.

Breaking change

This is not a breaking change. Successful build output is byte-identical (see validation), and no option, source map, or component behavior changes.

Related issue

Closes #6981. Follows up the investigation in #6850.

Problem and solution

A .on("error") listener only covers the stream it is attached to, so the handler could never intercept a minifier failure. For the one stream it did cover, Gulp 5 surfaces the error to the task anyway, so the handler's this.emit("end") changed nothing about the exit status. The result was one error printed twice: once by the handler through dutil.logError, once by Gulp's own reporter. Deleting the handler leaves Gulp's reporting as the single source of truth. dutil is still used elsewhere in the file, so the import stays.

Validation

Node 24.21.0, matching .nvmrc. Baseline is develop at adeb359.

Successful output is unchanged. gulp buildJS was run twice on develop to confirm the output is deterministic, then once with this change. All six files in dist/js (uswds.js, uswds.min.js, uswds.min.js.map, uswds-init.js, uswds-init.min.js, uswds-init.min.js.map) have identical sha256 before and after.

Failures still surface and exit nonzero. A scratch gulpfile loads the real tasks/javascript.js with a require hook that forces one stage to fail, and runs copyInit standalone, bundleMain standalone, and the exported compileJS (which is parallel(bundleMain, copyInit)). Output is redirected to a scratch directory. Two failure modes were exercised against both the develop file and this change:

In every case no .min.js is written and the task is reported as errored. The exit status is unchanged by this PR; only the duplicate log line goes away.

Existing checks.

• npm test: passes (128 Sass, 930 component, 71 tooling).
• npm run test:dist: 37 passing.
• eslint tasks/javascript.js: 0 errors. The one warning (unused eslint-disable directive on line 1) is pre-existing on develop. Prettier's trailing-comma diffs in this file are also pre-existing; tasks/ is outside prettier:check's scope, so they are left untouched to keep this diff minimal.

Coordination with #6974

#6974 changes jsPipeline's signature and the uglify() call a few lines below the deleted block. The two diffs do not overlap, but whichever merges second will need a trivial rebase. Either can go first.