Improve test coverage from 23% to 82%

[devin-ai-integration]

Summary

Improved test coverage for the RestSharp library from 23.07% baseline to 82.23% line coverage, exceeding the 80% threshold.

Changes

• RestSharp.Net7/: New SDK-style project (.NET 7) that references original RestSharp sources, with InternalsVisibleTo to expose internal OAuth classes for testing
• RestSharp.Tests.Coverage/: New xUnit test project with 677 passing tests across 25 test files

Coverage Results

Metric	Before	After
Line	23.07%	82.23%
Branch	28.38%	74.89%
Method	18.93%	92.77%

Test Files

• Core: RestRequestTests, RestClientTests, RestClientAdditionalTests, RestClientExecuteTests, RestResponseTests
• HTTP: HttpTests, HttpSyncTests, HttpResponseTests, HttpCookieTests
• Auth: AuthenticatorTests (Basic, Simple, OAuth2, NTLM), OAuth1AuthenticatorTests, OAuthTests, OAuthInternalTests
• Serialization: JsonSerializerTests, DotNetXmlTests, XmlAttributeDeserializerTests, XmlSerializerEdgeTests
• Extensions: StringExtensionsTests, MiscExtensionsTests, ReflectionExtensionsTests
• Other: ValidationTests, ParameterTests, FileParameterTests, FactoryAndEnumTests, AttributeAndContribTests, HttpUtilityTests, WebPairCollectionTests, CoverageBoostTests

Review & Testing Checklist for Human

• Run dotnet test in RestSharp.Tests.Coverage to verify all 677 tests pass
• Run coverage report to verify ≥80% line coverage: dotnet test /p:CollectCoverage=true /p:CoverletOutputFormat=cobertura /p:Include="[RestSharp]*"
• Spot-check a few test files to confirm tests are meaningful and not just exercising trivial paths

Notes

• The RestSharp.Net7 project references original source files via <Compile Include> wildcards rather than copying/modifying them
• Only modification to library code was adding InternalsVisibleTo attribute to enable testing of internal OAuth infrastructure classes
• Tests for HTTP execution methods use localhost:1 to trigger connection errors, exercising the error-handling code paths without needing a live server

Link to Devin session: https://app.devin.ai/sessions/7ebfa491af43427696d9415e079e06c6
Requested by: @bnob-git

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