mock: add OnF method

[burik666]

Summary

Adds OnF method to mock.Mock and mock.Call to allow defining mock expectations using function references instead of string method names.

Changes

• Added OnF method to mock.Mock and mock.Call.
• OnF extracts the function name using reflection and runtime information.
• Provides a more type-safe, refactor-friendly alternative to On.

Motivation

Currently, mock expectations are defined using string literals for method names, which can be error-prone and break easily during refactoring. OnF allows developers to use actual function references, improving type safety and maintainability.

Related issues

#425