Migrate jest to vitest

[korthout]

Jest requires --experimental-vm-modules for ESM support, relies on jest.unstable_mockModule for mocking ES modules, and needs ts-jest as a separate transform layer for TypeScript. This adds configuration complexity (NODE_OPTIONS, moduleNameMapper, tsconfig.test.json) and builds on unstable Node.js APIs.

Vitest has native ESM and TypeScript support, stable ESM mocking via vi.mock(), and requires minimal configuration. Its Jest-compatible API (describe/it/expect) means existing tests need few changes. It replaces three dev dependencies (jest, ts-jest, @types/jest) with one (vitest). Additionally, Vitest is much faster than jest which will help running tests in agent sessions.

node:test was considered as a zero-dependency alternative but requires rewriting all assertions from expect() to node:assert and needs a TypeScript loader. Given planned test coverage expansion, Vitest's lower migration cost and richer API make it the better choice.

[Copilot]

Pull request overview

Migrates the repository’s TypeScript ESM test setup from Jest/ts-jest to Vitest to reduce configuration complexity and improve test execution speed.

Changes:

• Add Vitest configuration and switch npm test scripts to vitest run.
• Remove Jest configuration and Jest-specific TypeScript typings/config references.
• Update existing tests to use Vitest APIs (vitest imports, vi.mock).

Reviewed changes

Copilot reviewed 7 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
vitest.config.ts	Adds Vitest test runner configuration (root under src, mock clearing).
package.json	Replaces Jest/ts-jest dependencies and scripts with Vitest equivalents.
jest.config.ts	Removes Jest configuration entirely.
tsconfig.test.json	Drops Jest typings/config inclusion for the test TS config.
tsconfig.json	Updates build excludes to avoid compiling the new Vitest config file.
src/test/utils.test.ts	Imports describe/it/expect from Vitest.
src/test/backport.test.ts	Imports describe/it/expect from Vitest.
src/test/git.test.ts	Replaces Jest ESM mocking with vi.mock and updates imports accordingly.

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[Copilot]

Pull request overview

Migrates the project’s TypeScript/ESM test setup from Jest + ts-jest to Vitest to reduce configuration complexity and improve test run performance.

Changes:

• Add vitest.config.ts and switch package.json test scripts/devDependencies to Vitest.
• Remove Jest configuration and Jest-specific TypeScript settings.
• Update existing tests to use Vitest APIs (vi.mock, describe/it/expect imports).

Reviewed changes

Copilot reviewed 7 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
vitest.config.ts	Introduces Vitest configuration (test root + mock behavior).
tsconfig.test.json	Removes Jest types/config include; adjusts test build output folder.
tsconfig.json	Replaces Jest config exclusion with Vitest config exclusion.
src/test/utils.test.ts	Imports Vitest test APIs.
src/test/git.test.ts	Replaces Jest ESM mocking with vi.mock and updates imports.
src/test/backport.test.ts	Imports Vitest test APIs.
package.json	Updates test scripts and replaces Jest/ts-jest deps with Vitest.
jest.config.ts	Removes Jest configuration.

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