fix: fail ESM builds after post-processing errors

[OskarEichler]

Summary

• set a nonzero process exit code when ESM post-processing fails
• add a regression test covering a rejected post-processing operation

Problem

The ESM build helper catches post-processing failures and logs them, but then exits successfully. CI or release automation can therefore accept incomplete or partially rewritten ESM output.

Breaking changes

ESM build commands that encounter a post-processing error now exit unsuccessfully instead of returning status 0. Runtime APIs and successful generated output are unchanged.

Verification

• regression test fails on current dev and passes with this change
• npm test: 94 suites, 795 tests passed
• npm run lint
• npm run babel: 184 files compiled and post-processed
• git diff --check