Improve test coverage to 90% and fix URL scheme driver bug

[Copilot]

This PR addresses the test coverage requirement (≥80%) and fixes a critical bug in URL scheme handling.

Coverage Improvements

• Increased coverage from 77.8% to 90.0% (exceeds 80% requirement)
• Added comprehensive test cases covering all major code paths:
  • New() function with both valid and nil URL parameters
  • Test() method with various connection failure scenarios
  • Edge cases for URL handling

Bug Fix: Incorrect Driver Name in sql.Open()

Problem: The Test() method was incorrectly using s.url.Scheme as the driver name in sql.Open(). This caused failures when PostgreSQL URLs used the alternative postgresql:// scheme instead of postgres://.

// Before (buggy)
db, err := sql.Open(s.url.Scheme, strings.TrimPrefix(s.url.String(), Scheme+"://"))

// After (fixed)  
db, err := sql.Open(Scheme, strings.TrimPrefix(s.url.String(), s.url.Scheme+"://"))

Impact: URLs with postgresql:// scheme would fail with "unknown driver" errors instead of properly attempting connections.

Test case added: Added test for postgresql:// URLs to verify the fix and prevent regression.

Test Coverage Breakdown

• Use(): 100% ✅
• New(): 100% ✅ (was 66.7%)
• Test(): 83.3% ✅ (was 80.0%)
• Total: 90.0% ✅ (was 77.8%)

All tests pass and code follows proper Go formatting standards.

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