Move test files to src/vs/sessions/test/e2e/

Keeps all E2E test code (mocks, runners, scenarios, extensions) in one
place, separate from production Sessions code. Updates import paths,
server path, and ESLint config.

Note: ESLint import-patterns warnings remain because the relative
imports from test/e2e/ depth don't match the restriction glob syntax.
This is cosmetic — the imports are valid TypeScript.

Co-authored-by: Copilot <[email protected]>