docs(http): add http.client Testing section with Flutter testing cookbook recommendation

[arowolojoshuao]

Summary

Adds a Testing section to the README that addresses the Flutter testing cookbook recommendation to use http.Client instances instead of static methods for better testability.

Changes

• Added Testing section after the composable client examples
• Included dependency injection pattern example with DataService class
• Added MockClient testing example with complete test case
• Referenced Flutter testing cookbook for best practices

Related Issue

Fixes #389

Testing

• Verified documentation formatting renders correctly
• Ensured code examples are syntactically correct
• Confirmed links to Flutter testing cookbook are valid