feat: use qs_dart in buildUrlString

[techouse]

👋 Hi there! I’m the author of qs_dart, a Dart port of qs for JavaScript.

I’ve given the buildUrlString utility method a thorough overhaul to boost its reliability and maintainability. Below is a summary of the key improvements:

🚀 Core Enhancements

• Stricter URL Validation
• Now raises an ArgumentError for malformed or non-HTTP(S) URLs
• Relies on Dart’s Uri.parse and basic checks - no more ad-hoc string assumptions
• Robust Query Encoding
• Switched to the qs_dart library for all query-string serialization
• Preserves existing multi-valued parameters and URL fragments

🛠️ New Dependencies

• qs_dart (v1.3.5+1) – Advanced query-string encoding
• validators (v3.0.0) – Optional URL format checks

✅ Expanded Test Coverage

Added new cases in test/utils/utils_test.dart to validate:

• Parameter edge cases
• null / empty maps, empty‐string values, overriding existing keys
• URL integrity
• Preserving fragments, rejecting invalid URLs
• Encoding nuances
• Special characters, reserved symbols (@, /), spaces vs. plus signs
• Data types & collections
• Numeric and boolean values, lists (including empty items), insertion-order guarantees

These tests ensure that buildUrlString behaves correctly across all common and edge scenarios.

[techouse]

@CodingAleCR I have added URL validation to the buildUrlString. If that's not desired, I can remove it.