Stringify Errors properly with --json flag

[tonyd33]

Summary

When running jest with the --json flag, the Errors from collectHandles in jest-core/src/runJest.ts is all passed into a JSON.stringify, which turns them into an empty object.

Users might expect or want the error message, name, and stack from the --json output possibly to collect data in the more structured JSON format — as opposed to plain stderr output — to help diagnose the source of the leaky handles.

Changes

Originally, I just iterated over the runResults.openHandles in runJest.ts:processResults to grab the error message, name, and stack from the Error, but adding a test for that was kind of awkward.

So I decided on adding a very basic serializeToJSON function that should replace JSON.stringify to handle stringifying Errors as well as leaving room to stringify other objects in the future if needed.

Test plan

• Added tests for serializeToJSON
  • Check that it's the same as JSON.stringify for regular objects
  • Check that it extends JSON.stringify by stringifying Errors properly

For a more e2e test, I added a temporary test to leak a handle, built the project, and ran the test:

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• :white_check_mark: login: tonyd33 / name: Tony (5f85233fb683962643adaa1d6423648fc24e7bac, 2cbba58bcadd9eef891ccf7cbc7b5d3f26b504fc)
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[SimenB]

https://github.com/jestjs/jest/releases/tag/v30.0.0-alpha.7

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