docs(readme): update coverage badges with latest stats

Update coverage badges to reflect recent test improvements:
- Test coverage: 75% → 75.49%
- Type coverage: 99.58% (unchanged)
- Add cumulative coverage badge: 87.53%

Reflects +23 tests and significant branch coverage improvements
in http-client.ts (+9.18% branches).

Author: jdalton

README.md

@@ -2,8 +2,9 @@
 
 [![Socket Badge](https://socket.dev/api/badge/npm/package/@socketsecurity/sdk)](https://socket.dev/npm/package/@socketsecurity/sdk)
 [![CI](https://github.com/SocketDev/socket-sdk-js/actions/workflows/ci.yml/badge.svg)](https://github.com/SocketDev/socket-sdk-js/actions/workflows/ci.yml)
-![Test Coverage](https://img.shields.io/badge/test--coverage-75%25-yellow)
+![Test Coverage](https://img.shields.io/badge/test--coverage-75.49%25-yellow)
 ![Type Coverage](https://img.shields.io/badge/type--coverage-99.58%25-brightgreen)
+![Cumulative Coverage](https://img.shields.io/badge/cumulative--coverage-87.53%25-brightgreen)
 
 [![Follow @SocketSecurity](https://img.shields.io/twitter/follow/SocketSecurity?style=social)](https://twitter.com/SocketSecurity)
 [![Follow @socket.dev on Bluesky](https://img.shields.io/badge/Follow-@socket.dev-1DA1F2?style=social&logo=bluesky)](https://bsky.app/profile/socket.dev)