Harden Jest setup to avoid leaking mocks between tests

[grmbyrn]

Title: Harden Jest setup to avoid leaking mocks between tests

Description:
Current tests override global.fetch in many places and may leak mocks across tests.

Acceptance criteria:

• Update jest.setup.ts to call jest.restoreAllMocks() / jest.resetAllMocks() appropriately between tests.
• Add afterEach cleanup hooks in tests that mock global state.
• Ensure CI runs tests with isolated environment.

Labels: testing
Priority: medium