GSOC-E2E: Add IME composition input test

[Geethegreat]

Issue:

Fixes #

Adds a test that verifies IME composition input leaves only the final character in the editor.

Demo:

e2e-ime.mp4

Changes:

• Clears the editor
• Opens a CDP session to simulate IME at the browser level
• Sends Input.imeSetComposition with intermediate Pinyin text nihao
• Verifies nihao appears in the editor as the composition in progress
• Sends a second Input.imeSetComposition with the final character 你好 and a replacement range covering nihao
• Fires compositionend to commit the composition
• Asserts the editor contains 你好 and does NOT contain nihao

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• has no typecheck errors (npm run typecheck)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123
• meets the standards outlined in the accessibility guidelines