Jest v30

[kubk]

Closes: #3920

• Updated Jest toolchain to v30. Fixed Jest 30 matcher breakage in tests: .toThrowError / .toBeCalled / .toBeCalledTimes
• Removed Jest validation warnings:
  -- Unknown option "coverageReporters": gone after upgrading to Jest 30, which supports it in project config.
  -- Unknown option "projects": gone by stopping the root config from loading packages/mobx’s aggregator as a child project.

[changeset-bot]

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Latest commit: a9ea856

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[kubk]

Before: https://github.com/mobxjs/mobx/actions/runs/22990167179/job/66749244732

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After: https://github.com/mobxjs/mobx/actions/runs/22993553384/job/66760016903

Same tests, no warning