refactor: create and export function handler types

[CorieW]

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[gemini-code-assist]

Code Review

This pull request performs a comprehensive refactoring across the v1 and v2 provider suites to introduce named exported types for all event handler signatures. By replacing inline function definitions with these specific types—such as OnWriteHandler, OnCallHandler, and BeforeUserCreatedHandler—the changes significantly improve code legibility and provide a better developer experience for users who need to explicitly type their handler functions. Minor adjustments were also made to import statements and internal type assertions to ensure consistency. I have no feedback to provide as the changes are purely structural and improve the overall type safety of the library.