chore(keyring-controller): replace Sinon with Jest mocks

[cryptodev-2s]

Explanation

References

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them

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Note

Low Risk
Test-only mocking changes and devDependency removal; no production logic is modified.

Overview
Removes sinon from @metamask/keyring-controller and migrates the remaining Sinon-based assertions/stubs in KeyringController.test.ts to Jest equivalents (jest.fn, toHaveBeenCalled*, jest.spyOn(...).mockRejectedValue).

This is a test-only refactor plus dependency cleanup (including yarn.lock), with no runtime behavior changes to the controller itself.

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