[Feature request]: Add Commonjs support

[sbayce]

What kind of issue is this?

Feature request

Summary

Currently the package is ESM-only. Every subpath in exports in package.json only declares an "import" condition — there's no "require" condition or CJS build output.

This means require('@opencoredev/email-sdk') (and any subpath) fails

Would it be possible to support CJS

Adapter or area

Core client

Minimal example or steps

No response

Relevant details

No response

Quick checks

• I removed API keys, tokens, and private customer data from this issue.
• I checked the docs or README first.
• I can help test a fix.