test: refactor TestBasicCredentials using table-driven tests

[iamamirsalehi]

Summary

This PR refactors the TestBasicCredentials function to use a table-driven testing approach. The goal is to improve test clarity, reduce duplication, and make it easier to extend with new cases in the future.

---

Changes

• ✅ Refactored TestBasicCredentials to table-driven format
• ✅ Combined basic auth and raw credentials checks under unified inputs
• ✅ Preserved all existing logic including the empty username case
• ✅ Replaced t.Fatalf with t.Errorf to allow multiple assertion failures per test run

---

Motivation

Table-driven tests are idiomatic in Go and make it easier to:

• Maintain and scale tests
• Add edge cases (e.g., empty password, special characters)
• Keep tests concise and DRY

---

Notes

Also included additional edge cases such as:

• Empty passwords
• Special characters
• Long strings
• Unicode characters

[ndyakov]

@iamamirsalehi thank you for the contribution!