Fix CI concurrent build exiting before static site generation

[seveibar]

Motivation

• Concurrent builds could call process.exit(1) from a worker onJobComplete when a fatal error occurred, which could terminate the process before post-build steps like --site completed.
• This caused an edge case where a config that uses prebuildCommand to generate board files (with includeBoardFiles globs and previewComponentPath/siteDefaultComponentPath set) would not produce dist/index.html or other outputs if any parallel build fatally failed.

Description

• Removed the immediate process.exit(1) inside the worker onJobComplete handler and no longer exit from inside the parallel job callback.
• Defer final exit handling until after all post-build tasks run and compute the exit code from hasFatalErrors and hasErrors vs ignoreErrors.
• Added a regression test tests/cli/build/build-ci-concurrency-site-fatal.test.ts that reproduces the scenario using a prebuildCommand that generates one successful and one fatally failing board file, and runs tsci build --ci --concurrency 4 to verify site output is still generated from successful builds.
• Changes are in cli/build/register.ts and the new test file under tests/cli/build/.

Testing

• Ran bun test tests/cli/build/build-ci-concurrency-site-fatal.test.ts and the test passed.
• Ran bunx tsc --noEmit for typechecking and it completed with no errors.
• Ran bun run format to format changes and it completed successfully.

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Codex Task

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