🧪 Tests: YAML config provider minimal tests

[jongalloway]

Add focused unit tests for YAML configuration provider.

Scope

• In tests/NLWebNet.Tests/Extensions, add a minimal test class validating:
  • AddYamlStream parses simple key/values and arrays
  • AddNLWebConfigurationFormats picks up config_retrieval.yaml when present
• Keep tests small, using in-memory streams and temp files

Acceptance Criteria

• Tests pass locally and in CI; coverage for basic YAML parsing and auto-discovery

Labels: tests, good first issue