Sonarqube Code Improvments

[marcschaeferger]

This pull request refactors and improves the session exchange and verification logic in main.go, focusing on better code organization, error handling, and reusability. The main changes include extracting logic into helper methods, improving error handling, and clarifying the session exchange flow.

Refactoring and code organization:

• Extracted session exchange logic from ServeHTTP into a new handleSessionExchange method, making the request flow clearer and easier to maintain.
• Moved repeated logic for building the original request URL, extracting headers, and extracting query parameters into dedicated helper methods (buildOriginalRequestURL, extractHeaders, extractQueryParams).

Error handling improvements:

• Centralized internal server error handling by introducing an internalServerError helper function and using it throughout the code for consistency. [1] [2] [3]
• Improved unauthorized access handling by returning a 401 Unauthorized response if session verification fails.

Constants and code clarity:

• Introduced constants for header names and error messages to avoid magic strings and improve maintainability.

These changes make the session handling logic more modular, readable, and robust against errors.