chore: migrate core/common to jest

[pearigee]

Migration to Jest & ts-jest

Successfully migrated the @google-cloud/common test suite from Mocha, Sinon, and c8 to Jest and ts-jest. Removed legacy configuration files (.mocharc.js, .nycrc), added coverage/ to .gitignore, and introduced jest.config.js to compile and execute tests on TypeScript files directly.

Key Unit Test Fixes & Isolation

• Hoisted Mocks: Standardized Jest mocks by resolving actual modules inside mock factories to avoid ReferenceError warnings due to Jest's top-level hoisting of jest.mock().
• Sandbox & State Leaks: Prevented state leakage across tests by deep-copying the shared authClient object (extend(true, {}, authClient, ...)) rather than mutating it directly.
• Asynchronous Leaks: Resolved a subtle issue in the authentication tests where a mocked authorizeRequest function resolved to undefined instead of the request options. This originally caused a TypeError in decorateRequest that was thrown asynchronously, bypassing done() and leaking as a failure into subsequent tests.

System Test Reliability & ESM Mocking

Note: System tests were broken on current versions of Node, this PR includes the following fixes:

• Port and Timeout Isolation: Allocated distinct ports (8118, 8119, 8120) to mock servers in system-test/common.ts to eliminate EADDRINUSE conflicts. Wrapped asynchronous callbacks in structured try...catch blocks to immediately close active servers and fail the test when assertions fail, preventing test runs from hanging.
• ESM Dynamic Import Workaround: Avoided requiring Node's --experimental-vm-modules CLI flag by mocking gaxios and teeny-request (which dynamically import node-fetch v3) at the top level of the system test file. The mocks redirect HTTP transport through Node's native global fetch API, enabling standard CommonJS execution in Jest.

[gemini-code-assist]

Code Review

This pull request migrates the core/common package's test suite from Mocha, Sinon, and Node's assert module to Jest. The changes include updating configuration files, dependencies, and rewriting tests to use Jest assertions and mocking utilities. The review feedback highlights that the migration is incomplete in test/service-object.ts due to remaining assert imports and calls. Additionally, the reviewer points out an anti-pattern of executing expect assertions inside a hoisted jest.mock factory function, recommending capturing the options in a local variable to assert on them within the test block instead.