fix: support relative container fetch URLs

[badmuriss]

Summary

• resolve relative URL paths against an internal container origin
• preserve existing behavior for absolute URLs and Request inputs
• add regression coverage for the relative path documented in the public API
• publish the fix as a patch release

Why

containerFetch('/api/data') is documented as supported, but the URL overload passed the relative path directly to the Request constructor. Since Request requires an absolute URL when no environment origin exists, the call failed with TypeError: Failed to parse URL from /api/data before reaching the container.

The URL overload now resolves its input against http://container. This supplies the origin required by Request while the existing TCP port fetch continues to route the request to the selected container port.

Fixes #28.

Testing

• npm run typecheck
• npm run typecheck:all
• npm run test:unit
• npm run lint
• npm run format:check
• npx changeset status

I also attempted npm run test. The local integration suite could not start its test containers and timed out in workerd with kj/timer.c++: overloaded: operation timed out, including the existing startup and HTTP fetch cases. The unit regression for this change passes.