chore: updating test files to not break when node is in v24

[joker23]

Node v24 makes os.platform immutable which causes the spyOn to fail.

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Note

Low Risk
Test-only changes with no production code paths affected.

Overview
Updates Electron and server-node platform info tests so they run on Node v24, where os.platform (and related APIs) are no longer spyable with jest.spyOn.

Both test files now jest.mock('os') with jest.fn() stand-ins for the os methods under test, beforeEach restores real implementations for tests that use the live OS, and the mock-data suites set mockReturnValue instead of spies. Mock suites also add afterEach(jest.clearAllMocks). Production SDK code is unchanged.

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[github-actions]

@launchdarkly/js-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 26365 bytes
Compressed size limit: 29000
Uncompressed size: 129044 bytes

[github-actions]

@launchdarkly/js-client-sdk-common size report
This is the brotli compressed size of the ESM build.
Compressed size: 38739 bytes
Compressed size limit: 39000
Uncompressed size: 212244 bytes

[github-actions]

@launchdarkly/js-client-sdk size report
This is the brotli compressed size of the ESM build.
Compressed size: 31978 bytes
Compressed size limit: 34000
Uncompressed size: 114243 bytes

[github-actions]

@launchdarkly/browser size report
This is the brotli compressed size of the ESM build.
Compressed size: 179579 bytes
Compressed size limit: 200000
Uncompressed size: 831422 bytes

[joker23]

@cursor review

[cursor]

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[devin-ai-integration]

✅ Devin Review: No Issues Found

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