feat: add coordinate mouse endpoint

[nicha16]

Summary

• Adds POST /tabs/:tabId/mouse for viewport CSS-pixel mouse actions (click, move, down, up).
• Validates coordinates, action, button, and click count; uses the existing per-user limit + tab lock path.
• Documents the endpoint in OpenAPI and regenerates openapi.json.
• Adds e2e coverage plus a test-client helper for coordinate mouse actions.

Test Plan

• npm run generate-openapi
• NODE_OPTIONS='--experimental-vm-modules' npx jest --runInBand --forceExit tests/unit/openapi.test.js
• NODE_OPTIONS='--experimental-vm-modules' npx jest --config jest.config.e2e.cjs --runInBand --forceExit tests/e2e/mouse.test.js