CORE-1996: Setup Jest configuration for CSS imports

[OpenStaxClaude]

Summary

This PR sets up Jest configuration to handle CSS imports, preparing the test environment for the migration from styled-components to plain CSS.

Changes

• Added identity-obj-proxy as a devDependency for mocking CSS imports in tests
• Configured Jest moduleNameMapper to handle CSS files with identity-obj-proxy
• Verified that all existing tests still run successfully

Related Issues

• Subtask: https://openstax.atlassian.net/browse/CORE-1996
• Parent: https://openstax.atlassian.net/browse/CORE-1777

Testing

• Ran full test suite: yarn test
• All tests pass (1 pre-existing snapshot failure unrelated to this change)

Notes

This is part of Phase 1 of the styled-components to plain CSS migration. The next step will be updating theme files to remove styled-components imports.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR prepares the Jest test environment for the upcoming migration away from styled-components by adding support for importing .css files during tests.

Changes:

• Added identity-obj-proxy as a devDependency to mock CSS imports in Jest.
• Updated jest.config.js with a moduleNameMapper rule for *.css.

Reviewed changes

Copilot reviewed 1 out of 3 changed files in this pull request and generated no comments.

File	Description
yarn.lock	Locks the newly added identity-obj-proxy dependency and its transitive dependencies.
package.json	Adds identity-obj-proxy to devDependencies for Jest CSS import mocking.
jest.config.js	Adds moduleNameMapper to map .css imports to identity-obj-proxy.

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