chore: upgrade dependencies, add testing setup & update Firebase handlers

[Iamdheeraj22]

This PR enhances the package by updating dependencies, improving Firebase integration, and adding support for unit and widget testing.

Changes

• Updated provider and firebase_auth to the latest compatible versions in pubspec.yaml.
• Added testing dependencies: flutter_lints, mocktail, and firebase_auth_mocks for unit and widget testing.
• Refactored Firebase handler and controller to align with the latest SDK changes.
• Updated package version and dependencies in the example app (pubspec.yaml).
• Upgraded Gradle configuration to match the latest Flutter compatibility requirements.

Outcome

• Improved package stability and compatibility
• Better testability with proper mocking support
• Cleaner and updated Firebase integration

These changes make the package more maintainable, testable, and aligned with current Flutter/Firebase standards.